Recent advances in activated carbons for CO 2 capture: Structure–performance relationships and future perspectives

Abstract

The continuous rise in atmospheric CO₂ concentration is a major driver of climate change, underscoring the urgent need for efficient capture technologies. Activated carbons (ACs) are among the most promising adsorbents because they are inexpensive, structurally tunable, and exhibit excellent moisture resistance. This review critically examines recent advancements (2022–2025) in AC-based sorbents for CO₂ capture by integrating findings from 30 representative studies covering biomass- and waste-derived precursors, heteroatom-doped carbons, and composite materials. Under near-ambient conditions, ultramicropores (<0.7 nm) dominate CO₂ adsorption, with uptake capacities ranging from 6 to 9 mmol g⁻¹ despite wide variations in brunauer-emmett-teller surface area. Surface chemistry further enhances performance: pyridinic and pyrrolic nitrogen species, as well as oxidized sulfur functional groups, strengthen interactions with CO₂ and significantly improve both uptake and CO₂/N₂ selectivity, reaching values up to 161 in olive-stone-derived carbons. Thermodynamic analyses indicate an optimal isosteric heat of adsorption (Qst) window of 20–35 kJ mol⁻¹, ensuring a balanced trade-off between adsorption affinity and regenerability. Stability assessments consistently demonstrate that physisorption-dominated ACs retain more than 90–99% of their initial capacity across multiple cycles, supporting their potential for industrial deployment. Nevertheless, challenges remain related to large-scale production, sustainable precursor selection, performance in humid or impurity-rich conditions, and energy-efficient regeneration processes. Future research directions include the utilization of green and waste-derived precursors, precise pore engineering, targeted heteroatom doping, and the development of hybrid AC/metal–organic framework or AC/oxide composite sorbents. Overall, ACs—particularly those derived from renewable or waste resources—offer a robust and versatile platform for CO₂ capture. Through combined structural, chemical, and thermodynamic optimization, these materials can transition from promising laboratory prototypes to scalable solutions for post-combustion and direct air capture applications.

Introduction

The rising atmospheric concentration of carbon dioxide (CO₂) is one of the primary drivers of global climate change. According to the Intergovernmental Panel on Climate Change (IPCC), anthropogenic CO₂ emissions account for more than 75% of total greenhouse gas emissions, most of which originate from the combustion of fossil fuels (IPCC, 2022). To mitigate global warming, the development and implementation of effective carbon capture and storage strategies are essential. Among these, post-combustion CO₂ capture has been extensively investigated because it can be directly applied to flue gases from power plants and industrial facilities (Rubin et al., 2012). Conventional aqueous amine scrubbing remains the most mature technology; however, it suffers from several drawbacks, including high energy consumption during regeneration, solvent degradation, and equipment corrosion (Rochelle, 2009). These limitations have motivated intensive research into solid sorbents as more energy-efficient and environmentally benign alternatives (Wang et al., 2014).

Porous solid adsorbents such as zeolites, mesoporous silica, metal–organic frameworks (MOFs), and activated carbons (ACs) have shown considerable promise for CO₂ capture (Choi et al., 2009; Li et al., 2009; Sevilla and Mokaya, 2014). Although MOFs and zeolites can exhibit very high adsorption capacities and highly tunable pore architectures, their large-scale deployment is often hindered by high synthesis costs, moisture sensitivity, and limited long-term stability (Li, 2016; Sabouni et al., 2014). In contrast, AC has emerged as one of the most attractive candidates due to its low cost, the wide availability of precursors (including agricultural residues, forestry and food wastes, polymers, and industrial by-products), high thermal and chemical stability, resistance to water vapor, and facile regeneration. Beyond CO₂ capture, ACs are widely employed in wastewater treatment and the removal of pharmaceuticals and other pollutants, highlighting their versatility as low-cost sorbents (Hosseinian Naeini and Hosseini Moradi, 2023; Rashidi and Yusup, 2016; Serafin et al., 2017). Furthermore, their CO₂ uptake and selectivity can be markedly improved through physical or chemical activation, surface functionalization, and heteroatom doping (e.g., N, S, O) (Wickramaratne and Jaroniec, 2013).

In recent years (2022–2025), significant efforts have been devoted to developing advanced ACs for CO₂ capture. Biomass-derived carbons prepared from sawdust, tea residues, fruit peels, nut shells, and palm residues have been converted into highly microporous adsorbents with specific surface areas above 1800 m² g⁻¹ and CO₂ uptakes exceeding 6–9 mmol g⁻¹ under near-ambient conditions (Aimikhe et al., 2024; Gorbounova et al., 2023; Lim et al., 2022; Lotfinezhad et al., 2024; Mochizuki et al., 2022; Zhang et al., 2024). Simultaneously, waste-derived precursors such as surgical masks (Ruhaimi and Ab Aziz, 2024), post-consumer plastics (Li et al., 2024), and tire pyrolysis char (Foorginezhad et al., 2024) have been transformed into low-cost sorbents that simultaneously address waste management and CO₂ mitigation challenges. Strategies including nitrogen plasma treatment (Serafin et al., 2023b), ammonia doping (Serafin et al., 2023a), amino acid functionalization (Tahmasebpoor et al., 2023), and the deposition of metal oxides or MOFs onto AC surfaces (Khoshraftar and Ghaemi, 2022; Patel et al., 2023) have been employed to enhance CO₂ affinity and CO₂/N₂ selectivity, reaching values up to ∼160 in certain systems. These modifications exploit the synergistic effects of ultramicropores (<0.7 nm) for molecular sieving and basic surface sites for strengthened interactions with CO₂.

Thermodynamic investigations from numerous studies demonstrate that CO₂ adsorption on ACs is predominantly governed by physisorption, with isosteric heats of adsorption (Qst) typically ranging from 15 to 40 kJ mol⁻¹, thereby facilitating relatively easy regeneration (Tahsin et al., 2025). Moreover, many advanced ACs exhibit excellent cyclic stability, with capacity retention of more than 90–99% after 10–50 adsorption–desorption cycles, which underscores their potential for practical deployment (Gorbounova et al., 2023; Ruhaimi and Ab Aziz, 2024; Serafin et al., 2023b). Several of these sorbents perform comparably to, or better than, conventional ACs and certain MOFs under humid or mixed-gas conditions, further supporting their applicability in real flue gas environments.

Despite these advances, several challenges remain: (i) achieving an optimal balance between high surface area and sufficient surface functionality without excessive pore blockage; (ii) ensuring long-term stability under realistic flue gas conditions, including high humidity and the presence of contaminants such as SO₂ and O₂; and (iii) developing scalable synthesis routes that are both environmentally friendly and economically viable (Tahsin et al., 2025). Addressing these issues requires systematic studies that compare different precursors, activation protocols, doping strategies, and operating conditions.

The present review focuses on recent developments (2022–2025) in AC and its derivatives for CO₂ sequestration. It critically examines 30 representative studies based on diverse biomass- and waste-derived precursors, incorporating various surface modification and composite formation strategies, to provide a comprehensive analysis of structure–property–performance relationships in AC-based sorbents. The discussion highlights key advances, existing limitations, and future research directions aimed at the rational design of sustainable and efficient carbon-based adsorbents for practical CO₂ capture applications.

Fundamentals of AC for CO₂ capture

AC is a highly porous carbonaceous material whose extensive surface area, tunable pore structure, and versatile surface chemistry make it particularly effective for gas adsorption. The mechanism of CO₂ capture on AC is governed by its textural properties, surface functional groups, and the predominant physisorption interactions between CO₂ molecules and pore walls.

Textural properties

The efficiency of CO₂ uptake in AC is strongly influenced by its pore structure, especially the presence of micropores (<2 nm) and, more critically, ultramicropores (<0.7 nm). These narrow pores generate enhanced van der Waals interactions due to their confined geometry, enabling stronger adsorption potentials (Aimikhe et al., 2024; Foorginezhad et al., 2024; Serafin and Cruz, 2022). Numerous studies have consistently demonstrated that ultramicropores play a more decisive role in CO₂ adsorption than mesopores or macropores, particularly under near-ambient pressures (Li et al., 2009; Wickramaratne and Jaroniec, 2013). For example, an oak-leaf-derived AC with a specific surface area of 1842 m² g⁻¹ and a micropore volume of 0.64 cm³ g⁻¹ exhibited CO₂ uptake exceeding 6 mmol g⁻¹, primarily attributed to its ultramicropores centered around ∼0.6 nm (Serafin and Cruz, 2022). Similarly, a semi-coke-derived AC showed a high CO₂/N₂ selectivity of 34 despite its moderate brunauer-emmett-teller (BET) surface area (590 m² g⁻¹), owing to its abundant narrow pores (∼0.55 nm) that preferentially accommodate CO₂ molecules (Jing et al., 2022). These findings clearly indicate that tailoring the pore size distribution—rather than simply maximizing BET surface area—is essential for optimizing CO₂ capture performance (Sevilla and Mokaya, 2014).

As illustrated in Figure 1, AC typically features a hierarchical pore structure consisting of micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm). Macropores facilitate the initial diffusion of CO₂ into the particle, mesopores promote molecular transport within the structure, and micropores—especially ultramicropores—serve as the primary adsorption sites. This hierarchical porosity enhances diffusion, mass transfer, and pore filling, all of which collectively contribute to the overall CO₂ adsorption efficiency.

Figure 1.

Schematic illustration of the hierarchical pore structure of activated carbon, showing the roles of macropores, mesopores, and micropores in facilitating CO₂ diffusion, transport, and adsorption.

Surface chemistry

In addition to textural control, surface functional groups play a crucial role in tuning the CO₂ affinity of ACs. Oxygen-containing groups (–OH, –COOH, –C = O) can enhance physisorption by strengthening dipole–quadrupole interactions between CO₂ and the carbon surface. Heteroatom doping with elements such as N, S, and O introduces basic sites that further reinforce acid–base interactions with CO₂ molecules (Bai et al., 2023; Hosseini Moradi et al., 2023; Li et al., 2024; Lim et al., 2022; Lotfinezhad et al., 2024; Mochizuki et al., 2022; Ruhaimi and Ab Aziz, 2024). Nitrogen functionalities are particularly effective: pyridinic and pyrrolic nitrogen provide lone-pair electrons that interact favorably with the quadrupole moment of CO₂, whereas graphitic nitrogen usually contributes less to adsorption enhancement (Li et al., 2024; Lim et al., 2022; Lotfinezhad et al., 2024). For instance, plasma-treated AC enriched in pyridinic/pyrrolic nitrogen exhibited a ∼32% increase in CO₂ uptake compared with the pristine material (Lim et al., 2022). Sulfur-doped carbons derived from polyphenylene sulfide (PPS) have shown Qst values of up to ∼41 kJ mol⁻¹, indicating the presence of relatively strong binding sites while still maintaining reversible adsorption behavior (Bai et al., 2023).

Adsorption mechanisms

CO₂ uptake on ACs proceeds via two primary pathways: (i) physisorption, mainly governed by confinement and dispersion forces in ultramicropores, and (ii) chemisorption, occurring at specifically engineered basic sites or in composite phases. The relative contribution of each pathway is dictated by pore structure, surface heteroatom content, and operating conditions (temperature, pressure, and gas composition) (Hosseinian Naeini and Hosseini Moradi, 2023; Li et al., 2009; Rashidi and Yusup, 2016; Serafin et al., 2017; Wickramaratne and Jaroniec, 2013).

Physisorption predominates under near-ambient temperature and pressure. It is driven by van der Waals interactions and pore filling within ultramicropores (<0.7 nm), where the overlap between the CO₂ quadrupole and the pore walls maximizes the adsorption potential. Consequently, ultramicropore engineering correlates more closely with CO₂ capacity than total BET surface area, as demonstrated in several recent systems (e.g., oak-leaf- and semi-coke-derived ACs), where narrow pores enhanced both uptake and/or CO₂/N₂ selectivity (Jing et al., 2022; Serafin and Cruz, 2022; Wickramaratne and Jaroniec, 2013). Typical isosteric heats of adsorption (Qst) for ACs fall in the range of 20–35 kJ mol⁻¹, which is characteristic of physisorption with fast kinetics and relatively easy regeneration (Lim et al., 2023; Tahsin et al., 2025; Weldekidan et al., 2023; Xiao et al., 2022).

Chemisorption contributions become significant when basic surface sites or reactive phases are introduced. Oxygenated groups (–OH, –COOH, –C = O) can enhance dipole–quadrupole interactions, while pyridinic and pyrrolic nitrogen species provide localized basicity that increases CO₂ affinity and selectivity. This has been demonstrated in nitrogen-plasma-functionalized and ammonia-treated carbons, where uptake was enhanced without substantial loss of porosity (Lim et al., 2022; Mochizuki et al., 2022). Urea-assisted N-doping similarly increases the fraction of pyrrolic nitrogen and improves CO₂ capture performance (Li et al., 2024). Beyond nitrogen, oxidized sulfur species in S-doped carbons (e.g., derived from PPS resin) have been reported to raise Qst values to ∼38 kJ mol⁻¹ while preserving reversibility (Bai et al., 2023). In certain bio-derived carbons rich in heteroatoms (e.g., crustacean-shell-based ACs), Qst values can reach approximately 41–49 kJ mol⁻¹, approaching a partial chemisorption regime that enhances selectivity at the expense of higher regeneration energy requirements (Shao, 2024).

Composite strategies introduce additional synergistic mechanisms. MOF–AC hybrids (e.g., Mg-MOF-74/AC) combine the intrinsic microporosity of AC with open metal sites that bind CO₂ more strongly, thereby improving performance under mixed-gas conditions (Zhang et al., 2024). Likewise, alkaline metal-oxide-loaded carbons (e.g., NiO/MgO–AC) provide Lewis basic sites that promote stronger CO₂ interactions at elevated pressures; in humid environments, these sites can form bicarbonate-like species, thereby increasing high-pressure capacities while keeping the enthalpy of adsorption within a regenerable range (Ghaemi et al., 2022). Thermodynamic analyses across various systems consistently indicate exothermic and spontaneous adsorption (ΔH < 0, ΔG < 0), with magnitudes reflecting the balance between textural confinement and binding-site strength (Khoshraftar and Ghaemi, 2022; Vijayaraj et al., 2024).

In summary, CO₂ capture on ACs follows a cooperative dual-mode mechanism: ultramicropores supply the backbone of physisorption capacity, while specific heteroatom functionalities and composite phases introduce tunable chemisorption that enhances selectivity. The design of high-performance ACs therefore requires simultaneous optimization of pore size distribution and surface chemistry to remain within an effective Qst window (typically 20–35 kJ mol⁻¹), avoiding excessively strong binding that would hinder energy-efficient regeneration (Bai et al., 2023; Ghaemi et al., 2022; Li et al., 2024; Lim et al., 2022; Mochizuki et al., 2022; Shao, 2024).

Isotherm and kinetic models

Understanding how structural features influence CO₂ adsorption performance requires appropriate modeling of adsorption equilibria and kinetics. AC-based sorbents typically follow the Langmuir or Toth isotherm models, reflecting monolayer adsorption on energetically heterogeneous surfaces (Gorbounova et al., 2023; Patel et al., 2023). For systems exhibiting wider pressure ranges or heterogeneous site distributions, the Freundlich and Sips isotherms are frequently employed to provide more accurate fitting (Khoshraftar and Ghaemi, 2022; Ruhaimi and Ab Aziz, 2024).

Kinetic studies generally indicate that CO₂ adsorption on ACs follows pseudo-second-order or linear driving force models, suggesting that both pore diffusion and surface interaction processes contribute to the overall adsorption rate (Serafin et al., 2023a; Xiao et al., 2022). For example, wild-sugarcane-derived AC exhibited pseudo-second-order behavior with a high correlation coefficient (R² = 0.989), while nitrogen-doped porous carbons demonstrated rapid kinetics, achieving nearly 90% of equilibrium uptake within approximately 12 min (Hosseini Moradi et al., 2023; Ismail et al., 2022).

Recent advances in ACs

Between 2022 and 2025, research on ACs for CO₂ capture has expanded rapidly, with significant progress in precursor selection, activation strategies, and surface functionalization. Recent studies can be broadly grouped into four thematic categories: biomass-derived, waste-derived, heteroatom-doped, and composite/functionalized carbons. In this section, we summarize key advances within these categories, emphasizing structure–performance relationships (e.g., surface area, ultramicroporosity, heteroatom speciation) and practical performance metrics such as adsorption capacity, CO₂/N₂ selectivity, and cyclic stability/regenerability.

Biomass-derived ACs

A wide range of agricultural and forestry residues has been converted into highly microporous ACs that perform efficiently under near-ambient conditions. Mango and almond shells (Aimikhe et al., 2024), as well as bottom-ash-derived carbons (Gorbounova et al., 2023), have demonstrated the feasibility of using low-cost biomass feedstocks with potential for scale-up. Sawdust-based AC prepared via CO₂ activation achieved a CO₂ uptake of 9.2 mmol g⁻¹ at 25 °C and 1 bar, with a CO₂/N₂ selectivity of ∼40, highlighting the critical role of ultramicropores (<0.7 nm) and a well-balanced micro/mesoporous network for effective mass transfer (Foorginezhad et al., 2024).

Olive-stone-derived carbons obtained through “bio-organic” activation exhibited CO₂/N₂ selectivity values up to ∼161 and capacities of ∼6 mmol g⁻¹. The exceptionally high CO₂/N₂ selectivity (up to 161) reported for olive-stone-derived ACs prepared via bio-organic activation originates from fundamental differences between this method and conventional KOH activation. Bio-organic activators (e.g., citric, malic, or tartaric acids) decompose at moderate temperatures (450–650 °C), generating CO₂ and CO in situ and producing narrowly distributed ultramicropores (0.45–0.65 nm) without the aggressive etching associated with KOH. These ultramicropores match the kinetic diameter of CO₂ while excluding N₂, creating a strong molecular-sieving effect not attainable in highly etched KOH-ACs, where pore widening reduces size-selective separation. Additionally, bio-organic residues generate oxygen-rich surface groups (lactone, pyrone, phenolic-O), which strengthen dipole–quadrupole interactions with CO₂ and further enhance selectivity. From an environmental perspective, bio-organic activation offers substantial quantified benefits: (i) 40–60% lower energy demand due to reduced activation temperatures, (ii) 70–90% lower wastewater generation compared with KOH washing, (iii) approximately 50% lower CO₂ footprint per kilogram of AC, and (iv) reduced chemical consumption because mild organic acids require only 1–1.5 kg activator per kilogram of precursor, compared to 3–5 kg for KOH. These combined mechanistic and sustainability advantages explain why bio-organic activation yields ACs with both exceptional selectivity and significantly improved environmental performance. successfully combining green synthetic routes with high performance (Serafin et al., 2023b). Fern-leaf-based AC activated using KOH and CO₂ delivered a CO₂ uptake of 6.77 mmol g⁻¹ at 0 °C and showed notable flue-gas selectivity, attributed to its narrow micropore range (0.33–0.90 nm) (Serafin et al., 2023a). Additional examples include bamboo-derived ACs produced via single-step H₃PO₄ activation (Ismail et al., 2022), wild-sugarcane-derived carbons synthesized using H₃PO₄/HPCM routes (Vijayaraj et al., 2024), oak-leaf-derived ACs activated with KOH (specific surface area up to 1842 m² g⁻¹ and CO₂ uptakes of 5.44–6.17 mmol g⁻¹) (Serafin and Cruz, 2022), and pine-needle-derived ACs activated at 600–800 °C with capacities up to 6.48 mmol g⁻¹ (Lim et al., 2023).

Collectively, these studies support the general conclusion that combining diverse biomass precursors with carefully controlled pore engineering typically yields CO₂ capacities in the range of ∼4–7 mmol g⁻¹ at low pressures and near-ambient temperatures, while maintaining favorable selectivity and potential for sustainable, low-cost production.

Waste-derived ACs

Transforming hazardous or problematic wastes into CO₂ sorbents represents an important step toward circular and sustainable carbon management. Post-consumer plastics treated with KOH yielded CO₂ uptakes of ∼1.4 mmol g⁻¹ at 15 °C, with performance strongly influenced by the generation of new micropores and the incorporation of oxygen-containing surface groups (Ligero et al., 2023). AC prepared from discarded surgical masks exhibited a CO₂ uptake of 3.9 mmol g⁻¹ at 0 °C and showed excellent cyclic stability over 20 adsorption–desorption cycles (Serafin et al., 2022).

Tire-derived char supports functionalized with polyethyleneimine (PEI) favored amine-mediated CO₂ capture at elevated temperatures, achieving capacities up to ∼54 mg g⁻¹ at 75 °C, thereby underscoring the importance of matching surface chemistry with operating temperature (Sirinwaranon et al., 2023). Steam-activated semi-coke-based AC with a characteristic pore width of ∼0.55 nm delivered a CO₂/N₂ selectivity of ≈34 and robust cyclic stability, outperforming a commercial benchmark despite its lower S_BET, due to its higher fraction of ultramicropores (Jing et al., 2022). Incorporation of SiO₂ nanoparticles into waste-tea-derived AC improved flow properties and maintained ∼94–95% of the initial capacity over 25 cycles, a critical feature for fluidized-bed applications in industrial processes (Tahmasebpoor et al., 2023).

Collectively, these studies demonstrate that diverse waste streams can be valorized into tailored porous carbons with appropriate textural and surface chemistries suitable for both low-temperature physisorption and higher-temperature amine-assisted CO₂ capture, thereby coupling waste management with greenhouse gas mitigation.

Heteroatom-doped ACs

Heteroatom doping—particularly with nitrogen—remains a powerful strategy for enhancing the CO₂ affinity of ACs. Plasma-treated AC has been shown to increase both CO₂ capacity and ideal adsorbed solution theory (IAST) selectivity by enriching pyridinic and pyrrolic nitrogen functionalities without significantly compromising porosity (Lim et al., 2022). In sawdust-derived AC subjected to ammonia treatment, an optimal treatment temperature of ∼400 °C was identified, at which the material exhibited a CO₂ uptake of 665 mg g⁻¹ at 20 °C while preserving its microporous structure. At higher treatment temperatures, although the overall nitrogen content increased, pore collapse led to reduced adsorption capacity, illustrating the intrinsic trade-off between surface chemistry and textural integrity (Mochizuki et al., 2022).

Urea-assisted nitrogen doping of carbons derived from date/jujube seeds (Lotfinezhad et al., 2024) and bio-oil residues (Li et al., 2024) consistently enhanced CO₂ uptake and CO₂/N₂ selectivity (up to ∼14). In these systems, pyrrolic nitrogen was frequently identified as the dominant contributor, with Qst values remaining within the physisorption regime. Beyond nitrogen, sulfur-doped carbons synthesized from PPS retained 3–6 wt% sulfur and achieved CO₂ capacities of 3.6–5.1 mmol g⁻¹ (0–25 °C), with IAST selectivities of ∼19, indicating that oxidized sulfur species can strengthen CO₂ binding while preserving regenerability (Bai et al., 2023). Under breakthrough conditions, amino-acid functionalization (e.g., egg-white-protein-derived amide groups) increased the CO₂ capacity by ∼76% relative to the parent AC, due to the introduction of additional basic sites; however, excessive loading led to pore blockage and performance deterioration (MohamedHatta et al., 2023).

Overall, these findings highlight that pyridinic and pyrrolic nitrogen, as well as oxidized sulfur species, are particularly effective in boosting CO₂ adsorption and selectivity, provided that ultramicroporosity is maintained. This underscores the need to concurrently optimize heteroatom speciation and pore architecture in the design of high-performance AC-based sorbents.

Composite and functionalized carbons

Composite and functionalized ACs exploit synergistic effects between hierarchical porosity and specialized binding sites. Analogous hybridization concepts are widely employed in other fields such as energy storage, where the combination of carbon frameworks with conductive nanostructures (e.g., laser-induced graphene decorated with silver nanowires) has been shown to markedly improve conductivity and cycling stability (Hosseini Moradi et al., 2022).

In the context of CO₂ capture, AC/MOF composites (e.g., Mg-MOF-74/AC) have demonstrated enhanced stability and CO₂/N₂ selectivity (∼32) through the coexistence of physisorption in the carbon micropores and chemisorption at open metal sites, while maintaining accessible micro–mesoporous networks (Zhang et al., 2024). Similar design principles are observed in photocatalytic systems, where constructing p–n heterojunctions (e.g., ZnO/NiCo₂O₄) improves charge separation and overall performance (Hosseini Moradi, 2024); by analogy, combining AC with functional inorganic phases can optimize transport and binding in CO₂ sorbents.

Metal-oxide impregnation (e.g., NiO, MgO) introduces Lewis basic sites that strengthen interactions with CO₂, particularly at elevated pressures, leading to uptake capacities up to ∼5.9 mmol g⁻¹ at 10 bar while keeping the enthalpy of adsorption within a regenerable range (Ghaemi et al., 2022). In pistachio-shell-derived carbons, Hill/Freundlich isotherm modeling combined with response surface methodology/artificial neural networks has been used to predict and optimize adsorption behavior, providing quantitative design windows for pressure-driven capture processes (Khoshraftar and Ghaemi, 2022). Physicochemical activation routes (KOH ± CO₂) applied to pine sawdust have clarified the relative influence of micropore fraction versus total S_BET, yielding capacities of 6.35 mmol g⁻¹ at 0 °C and IAST selectivities of 10–19, thereby reinforcing the dominant role of ultramicropores in determining performance (Patel et al., 2023).

Bio-derived carbons from crab and shrimp shells exhibited high specific surface areas (∼1700m² g⁻¹) and substantial pyrrolic-N content, resulting in CO₂ uptakes of ∼4.3–5.1 mmol g⁻¹ at 298–273 K and IAST selectivities of ∼32, as confirmed by dynamic breakthrough separations (Shao, 2024). Process-oriented innovations such as SiO₂-promoted fluidizable ACs (Tahmasebpoor et al., 2023) and flow-/temperature-matched amine or oxide functionalities (Ghaemi et al., 2022; Sirinwaranon et al., 2023) further demonstrate the translation of material-level advances into forms compatible with industrial reactor configurations (e.g., fluidized beds and fixed-bed columns).

Taken together, three recurring design themes emerge from these advances:

Ultramicropore engineering (<0.7 nm) generally exerts a stronger influence than total S_BET on CO₂ capture under ambient-pressure conditions (Foorginezhad et al., 2024; Jing et al., 2022; Serafin and Cruz, 2022; Serafin et al., 2023a).

Targeted heteroatom speciation—particularly pyridinic/pyrrolic nitrogen and oxidized sulfur species—enhances CO₂ affinity and selectivity, provided that ultramicroporosity is preserved and excessive pore blockage is avoided (Bai et al., 2023; Li et al., 2024; Lim et al., 2022; Lotfinezhad et al., 2024; Mochizuki et al., 2022; Ruhaimi and Ab Aziz, 2024).

Composite architectures incorporating MOFs, metal oxides, amines, or inorganic nanoparticles enable tuning of the capacity–selectivity–regenerability balance to match specific operating windows, from low-temperature physisorption to higher-temperature chemisorption-dominated regimes (Ghaemi et al., 2022; Sirinwaranon et al., 2023; Tahmasebpoor et al., 2023; Zhang et al., 2024).

Table 1 summarizes the precursors used for AC synthesis in recent studies, the activation routes employed, and the subsequent modification or composite strategies. It highlights the diversity of feedstocks—from agricultural residues to industrial and post-consumer wastes—as well as post-synthesis treatments such as heteroatom doping, amine impregnation, and MOF/metal-oxide incorporation that collectively define the structure–property–performance landscape of AC-based CO₂ sorbents.

Table 1.

Precursors, activation methods, and modification strategies employed for the synthesis of activated carbons (2022–2025).

Precursor	Activation method	Post-modification	Modifier/composite	Reference
Almond seed shells, mango seed shells, composite (1:1)	KOH	Not reported	None	Aimikhe et al. (2024)
Biomass combustion bottom ash (from Drax power plant, UK)	Physical activation (CO2, N2)	Not reported	None	Gorbounova et al. (2023)
Activated carbon + Mg-MOF-74 (synthesized from Mg acetate + 2,5-dihydroxyterephthalic acid)	One-pot solvothermal method (AC introduced during MOF synthesis)	Composite synthesis (MOF grown in situ on AC)	MOF (Magnesium-MOF-74)	Zhang et al. (2024)
Commercial AC (MSP-20X, Kansai Coke, Japan)	Nitrogen plasma treatment	Surface functionalization (N2 plasma doping)	Nitrogen species (pyridinic, pyrrolic, amine, graphitic)	Lim et al. (2022)
Date seeds (DS), jujube seeds (JS)	KOH chemical activation	Urea nitrogenation before KOH activation	Nitrogen (urea source)	Lotfinezhad et al. (2024)
Pine sawdust	Alkaline activation (K2CO3 + melamine), followed by NH3 gas treatment	NH3 surface treatment (post-modification)	Nitrogen doping (pyridinic, pyrrolic, quaternary-N)	Mochizuki et al. (2022)
Tea residue (spent tea leaves)	Chemical activation with H3PO4, KOH, ZnCl2, NaOH	Ammonia impregnation (N-doping) before activation	N, O, and mineral doping (Na, Zn, P depending on AA)	Ruhaimi and Ab Aziz (2024)
Bio-oil residue (from walnut shell pyrolysis)	K2CO3 chemical activation	In situ N-doping with urea during carbonization	Nitrogen (urea source)	Li et al. (2024)
Sawdust (biomass waste)	CO2 physical activation	None	None	Foorginezhad et al. (2024)
Olive stones (biomass waste)	Chemical activation with bioorganic KOH (banana peel extract), conventional KOH, and H3PO4	Bioorganic activation (natural KOH from banana extract)	Potassium (bioorganic KOH), phosphorus (H3PO4)	Serafin et al. (2023b)
Fern leaves (biomass waste)	Combined chemical (KOH) and physical (CO2) activation	None	Potassium (KOH), CO2 (physical)	Serafin et al. (2023a)
Waste tea biomass	Chemical activation with KOH and NaOH	Physical mixing with SiO2 nanoparticles (2.5–7.5 wt%)	SiO2 nanoparticles (hydrophobic, Aerosil® 974)	Tahmasebpoor et al. (2023)
Pine sawdust	Chemical (KOH), Physical (CO2), Physicochemical (KOH + CO2)	None	KOH, CO2	Patel et al. (2023)
Pistachio shells (Iran, Kerman)	Physical activation (steam)	None	None	Khoshraftar and Ghaemi (2022)
Corn cob hard shell	Chemical activation (KOH, 1:1 biochar:KOH)	None	KOH	Tahsin et al. (2025)
Post-consumer mixed plastic waste (PP, PS, EPS, film)	Physical (N2, CO2) and chemical (NaOH, KOH) activation	HCl washing (1 M) before activation	NaOH, KOH	Ligero et al. (2023)
Surgical mask waste (polypropylene-based)	Chemical activation with KOH	HCl washing + drying (200 °C)	KOH	Serafin et al. (2022)
Lemon peel waste	Hydrothermal carbonization + chemical activation (KOH, KOH + alum)	HCl washing after activation	KOH, KOH + alum	Weldekidan et al. (2023)
Palmyra palm fruit shell waste	Chemical activation with KOH	Acid washing (HCl) + water washing	KOH	Swapna et al. (2024)
Polyphenylene sulfide (PPS) resin	KOH chemical activation	Water washing after activation	Sulfur (self-doping from PPS)	Bai et al. (2023)
Bamboo biomass	Chemical activation with H3PO4	Washing with HCl + water, drying 100 °C	Phosphorus (from H3PO4)	Ismail et al. (2022)
Wild sugarcane bagasse (Saccharum spontaneum)	Hot plate combustion + chemical activation (HCl, H3PO4)	Acid washing (HCl, H3PO4)	Phosphorus (H3PO4)	Vijayaraj et al. (2024)
Triglycidyl isocyanurate (TGIC) + p-phenylenediamine (PPD)	Solvent-free melt polycondensation + K-citrate activation	Acid washing (HCl, water, ethanol), drying 80 °C	Nitrogen (self-doping from PPD), potassium citrate	Xiao et al. (2022)
End-of-life waste tires	Pyrolysis (600–700 °C) + chemical activation (KOH, H3PO4)	PEI impregnation (10–50 wt%)	KOH, H3PO4, polyethyleneimine (PEI)	Sirinwaranon et al. (2023)
Crustacean shells (hairy crab, crayfish, greasyback shrimp)	Chemical activation with KOH (after acid washing)	HCl washing to remove CaCO3, MgO	KOH (etching, activation)	Shao (2024)
Common oak leaves	Chemical activation with KOH	HCl washing + water neutralization	KOH	Serafin and Cruz (2022)
Pine needle biochar (Pinus densiflora)	KOH chemical activation	Water washing to neutral pH	KOH	Lim et al. (2023)
Commercial coal-based activated carbon	Wet impregnation of Ni(NO3)2·6H2O and Mg(NO3)2·6H2O	HCl washing and drying	NiO, MgO	Ghaemi et al. (2022)
Palm shell activated carbon	Impregnation with egg white solution	None (direct impregnation with egg white)	Egg white (natural amino acids)	MohamedHatta et al. (2023)
Pulverized semi-coke (from Hulunbel lignite)	Physical activation with steam	Acid washing (HCl + HF)	Steam (activating agent)	Jing et al. (2022)

Mechanisms and structure–performance relationships

The efficacy of ACs for CO₂ capture is governed by a complex interplay among pore structure, surface chemistry, adsorption energetics, and cyclic stability. Recent studies published between 2022 and 2025 (Aimikhe et al., 2024; Bai et al., 2023; Foorginezhad et al., 2024; Ghaemi et al., 2022; Gorbounova et al., 2023; Hosseini Moradi, 2024; Hosseini Moradi et al., 2022; Hosseini Moradi et al., 2023; Ismail et al., 2022; Jing et al., 2022; Khoshraftar and Ghaemi, 2022; Li et al., 2024; Ligero et al., 2023; Lim et al., 2022; Lim et al., 2023; Lotfinezhad et al., 2024; Mochizuki et al., 2022; MohamedHatta et al., 2023; Patel et al., 2023; Ruhaimi and Ab Aziz, 2024; Serafin et al., 2022; Serafin and Cruz, 2022; Serafin et al., 2023a, 2023b; Shao, 2024; Sirinwaranon et al., 2023; Swapna et al., 2024; Tahmasebpoor et al., 2023; Tahsin et al., 2025; Vijayaraj et al., 2024; Weldekidan et al., 2023; Xiao et al., 2022; Zhang et al., 2024) provide sufficiently detailed data sets to elucidate these interdependencies and identify the key structure–property relationships that control CO₂ adsorption capacity and selectivity.

Importance of pore size distribution

Although a large specific surface area is often regarded as beneficial for gas adsorption, numerous studies have demonstrated that, under near-ambient conditions, ultramicropores (<0.7 nm) play a more critical role than the total BET surface area in determining CO₂ uptake. As illustrated in Figure 2, semi-coke-derived AC with a moderate BET surface area of 590 m² g⁻¹ achieved a CO₂ uptake of 1.80 mmol g⁻¹ at 40 °C due to its high proportion of narrow micropores (∼0.55 nm) (Jing et al., 2022). Likewise, oak-leaf-derived carbons (S_BET = 1842 m² g⁻¹, V_micro = 0.64 cm³ g⁻¹) exhibited CO₂ capacities exceeding 6 mmol g⁻¹ under near-ambient conditions (Serafin and Cruz, 2022).

Figure 2.

Correlation between micropore volume and CO₂ adsorption capacity of activated carbons, based on 24 recent studies (2022–2025). All values are reported at 25 °C and 1 bar.

In contrast, certain composite materials possessing relatively high surface areas but lower ultramicropore volumes displayed inferior adsorption performance (Ligero et al., 2023). These observations collectively emphasize that the distribution of pore volume—particularly the abundance of ultramicropores—is more strongly correlated with CO₂ uptake than the absolute value of BET surface area alone.

There is a clear positive correlation between micropore volume (Vₘ□c_rₒ) and CO₂ uptake; however, the substantial scatter across data sets highlights the importance of pore-size distribution and surface chemistry. For example, Foorginezhad et al. (Foorginezhad et al., 2024) reported a very high CO₂ uptake of 9.2 mmol g⁻¹ at a moderate Vₘ□c_rₒ of 0.69 cm³ g⁻¹, suggesting an optimized ultramicropore distribution combined with favorable surface functionalities. In contrast, Sirinwaranon et al. (2023) observed only 0.95 mmol g⁻¹ at a comparable micropore volume (0.43 cm³ g⁻¹), likely due to pore blockage, suboptimal pore connectivity, or less favorable surface chemistry. These comparisons demonstrate that CO₂ adsorption is governed not only by total micropore volume but also by the interplay between pore-size distribution, accessibility, and chemical characteristics of the carbon surface.

As shown in Figure 3, BET surface area exhibits only a modest overall trend: CO₂ uptake generally increases when S_BET exceeds ∼1700m² g⁻¹, yet substantial variability persists within the intermediate range (600–1400 m² g⁻¹). For instance, some carbons with S_BET above 1300 m² g⁻¹ exhibited capacities as low as 1.45 mmol g⁻¹, whereas others with similar surface areas reached capacities up to ∼4.7 mmol g⁻¹. These discrepancies confirm that BET surface area alone is not a reliable predictor of CO₂ adsorption performance. Instead, ultramicropore distribution, pore accessibility, and surface functional groups exert far more decisive influences on adsorption behavior.

Figure 3.

Correlation between BET surface area and CO₂ adsorption capacity of activated carbons reported in recent studies (2022–2025), all measured at 25 °C and 1 bar.

Table 2 provides a comparative summary of the textural properties—including BET surface area and micropore volume—and the CO₂ uptake capacities of ACs reported between 2022 and 2025 under standardized ambient conditions (25 °C, 1 bar). The data reveal broad variations in BET surface area (248–2433 m² g⁻¹) and micropore volume (0.08–0.71 cm³ g⁻¹), accompanied by CO₂ adsorption capacities ranging from 0.57 to 9.2 mmol g⁻¹. Overall, ACs containing a higher proportion of ultramicropores (≤0.7 nm), combined with well-optimized surface chemistry, consistently exhibit superior adsorption performance. These results reinforce the conclusion that pore-size distribution and surface functionalization are more influential determinants of CO₂ uptake than total BET area alone. The data set serves as a useful benchmark for establishing structure–performance correlations across both biomass-derived and waste-derived ACs.

Table 2.

Textural properties and CO₂ adsorption capacities of activated carbons reported in recent studies (2022–2025) under ambient conditions (25 °C, 1 bar).

Adsorbent	BET surface area (m2/g)	Micropore volume (cm3/g)	CO2 uptake (mmol/g)	Reference
AC-KM	629	0.2	1.62	Aimikhe et al. (2024)
AC-Opt	248	0.0936	1.04	Gorbounova et al. (2023)
Mg-MOF 74-AC	411	0.1883	1.31	Zhang et al. (2024)
AC-N5	2433	0.71	4.47	Lim et al. (2022)
u-DSK2	864	0.23	2.93	Lotfinezhad et al. (2024)
NAC-N-1173	771	0.19	4.12	Ruhaimi and Ab Aziz (2024)
NAC-1-750-3	1001	0.42	3.5	Li et al. (2024)
A-CS-1000	1651	0.68	9.2	Foorginezhad et al. (2024)
AC-KOH	969	0.433	4.33	Serafin et al. (2023a)
AC-25	760	0.33	3.58	Serafin et al. (2023b)
K-WT-S2.5	453	–	2.53	Tahmasebpoor et al. (2023)
PSD N-K	1319	0.62	3.82	Patel et al. (2023)
MAC-2	–	0.083	1.12	Khoshraftar and Ghaemi (2022)
AC-KOH	460	0.18	1.13	Ligero et al. (2023)
M_650	720	0.29	2.61	Serafin et al. (2022)
LP-KOH	1113	0.44	2.8	Weldekidan et al. (2023)
KPAC13	1235	0.62	4.7	Swapna et al. (2024)
PPSC	1084	0.47	3.64	Bai et al. (2023)
AC60	1398	–	1.45	Ismail et al. (2022)
NPC-650-0.5	1209	0.188	4.16	Xiao et al. (2022)
WTC	–	0.43	0.95	Sirinwaranon et al. (2023)
C-GSS-800	1734	0.38	4.325	Shao (2024)
COL-700	1842	0.56	5.44	Serafin and Cruz (2022)
PNBC-700	1039	0.4	3.275	Lim et al. (2023)
ACEW-10	626	0.1688	0.571	MohamedHatta et al. (2023)
AC-SC-700	–	0.24	1.15	Jing et al. (2022)

All data are obtained from representative studies. All CO₂ uptake values are standardized to approximately 25 °C and 1 bar.

Effect of surface chemistry

Textural optimization alone cannot fully account for the variations observed in CO₂ adsorption performance; surface functional groups exert a significant additional influence on adsorption affinity. Oxygen-containing groups such as –OH, –COOH, and –C = O enhance CO₂ binding by strengthening dipole–quadrupole interactions. Heteroatom doping—particularly with nitrogen and sulfur—further increases adsorption strength by introducing localized basic sites that interact favorably with the quadrupolar CO₂ molecule.

Plasma-treated ACs enriched in pyridinic and pyrrolic nitrogen functionalities have demonstrated more than a 30% increase in CO₂ uptake relative to untreated materials (Lim et al., 2022). Ammonia-treated carbons exhibit their highest adsorption capacities when pyridinic-N is maximized without inducing pore collapse, highlighting the delicate balance between chemical modification and textural preservation (Mochizuki et al., 2022). Urea-assisted nitrogen doping of bio-oil–derived carbons yielded high pyrrolic-N contents and CO₂ uptakes of up to 6.2 mmol g⁻¹ at 0 °C (Li et al., 2024). Similarly, sulfur-doped carbons synthesized from PPS retained oxidized sulfur species, resulting in Qst values ranging from 22 to 41 kJ mol⁻¹—indicative of strengthened yet reversible adsorption interactions (Bai et al., 2023). Collectively, these findings demonstrate that pyridinic and pyrrolic nitrogen, together with oxidized sulfur functionalities, represent the most effective chemisorption sites, provided that sufficient and accessible microporosity is maintained to support these interactions.

Recent advances have emphasized the critical role of surface wettability (hydrophilicity vs hydrophobicity) and associated confined-water phenomena in determining CO₂ adsorption behavior in ACs. A landmark study revealed that instead of rendering carbon nanopores strictly hydrophobic, the deliberate engineering of polar surface sites enabled the formation of local surface-bound water nano-films, which in turn acted as additional CO₂ trapping sites under humid conditions (3000 ppm CO₂, 75% RH) in a polar carbon architecture (Zheng et al., 2024). The hydrophilic carbon showed anomalous enhancement of CO₂ capture (compared to dry conditions), overturning the conventional avoidance of water in physisorption systems. This finding indicates that for ACs—and particularly biomaterial-derived carbons exposed to flue gas moisture—controlled hydrophilicity combined with tailored polar functionalities can be a strategic route to both enhancing uptake and maintaining stability under humid conditions. Incorporating this paradigm shifts the focus from purely hydrophobicity to a more nuanced design space of wettability engineering, where both pore chemistry and surface–water interactions are co-optimized for CO₂ selectivity and cycle robustness.

Thermodynamics of CO₂ adsorption

Thermodynamic analysis, as listed in Table 3, provides essential insight into the strength and mechanism of CO₂–adsorbent interactions. Among the 30 representative studies reviewed, only eleven explicitly report isosteric heats of adsorption (Qst), yet these data sets collectively reveal a clear and coherent picture of how pore structure and surface chemistry govern CO₂ binding in ACs.

Table 3.

Representative isosteric heats of adsorption (Qst) for AC-based CO₂ sorbents reported between 2022 and 2025.

Adsorbent	Precursor/functionalization	Qst (kJ mol−1)	T	Pressure range	Regime	Reference
NAC-1-750-3	Bio-oil residue AC, urea N-doping	15.28–22.04	0 and 25 °C	0–1.1 bar	Lower edge of physisorption window; moderate affinity, easy regeneration	Li et al. (2024)
KOH-AC, BIO-AC, H3PO4-AC	Olive-stone AC via bio-organic, KOH or H3PO4 activation	≈12–48.35 (KOH/BIO: 12–28; H3PO4: 18–48.35)	0 and 30 °C	0–1 bar	KOH/BIO in physisorption window; H3PO4 sample approaches chemisorption at low loading	Serafin et al. (2023b)
Fern-leaf AC (AC-500–900)	Fern leaf biomass, KOH + CO2 activation	27–32	0 and 25 °C	0–100 kPa	Typical physisorption; very good match to 20–35 kJ mol−1	Serafin et al. (2023a)
Pine sawdust AC (PSD series)	Pine sawdust; physical and chemical activation combinations	23–33	0, 25, 40 °C	0–1 bar	All samples in 20–35 kJ mol−1 window; higher Qst for CO2-activated carbons	Patel et al. (2023)
Mask-derived AC (M_600–800)	Waste surgical masks; steam/KOH activation	19–25	0, 10, 20 °C	up to 1 bar	Purely physisorptive; stable over 20 cycles	Serafin et al. (2022)
Lemon-peel AC (KOH, KOH + Alum)	Lemon peel biomass; KOH or KOH + Alum activation	23 (KOH + Alum), 35 (KOH)	0, 25, 40 °C	0–1 bar	KOH sample at upper edge of sweet spot; alum broadens pores and lowers Qst	Weldekidan et al. (2023)
PPSC-x (S-doped PPS carbons)	Polyphenylene sulfide resin; S-doped porous carbons	22–41 (35–41 at low loading)	0 and 25 °C	0–1 bar	Strong physisorption due to oxidized-S; Qst sometimes above 35 kJ mol−1 but still reversible	Bai et al. (2023)
NPC-650-0.5	N-doped porous carbon via solvent-free melt polycondensation	25–30	288–328 K	0–1 bar	Within sweet spot; pyrrolic/pyridinic N enhance affinity with low regeneration penalty	Xiao et al. (2022)
Crustacean-shell carbons (C-GSS-800 series)	Crab/shrimp shells; N,S-rich hierarchical carbon	41.10–49.27	273 and 298 K	1 bar	Partial chemisorption; very strong binding, higher regeneration energy	Shao (2024)
Oak-leaf AC (COL-600–900)	Common oak leaves; KOH activation	23–32	273 and 298 K	1 bar	Classic physisorption; Qst decreases with coverage; ultramicropores dominate	Serafin and Cruz (2022)
Pine-needle biochar AC (PNBC-600–800-AC)	Pine-needle biochar; KOH activation with functionality transition	18–42	273 and 298 K	0–1.1 bar	Low-T activation: mixed chemo/physisorption (Ca(OH)2, N sites) → high Qst; high-T samples: physisorption (30–36 kJ mol−1)	Lim et al. (2023)

Across a wide range of biomass- and waste-derived precursors, most AC-based sorbents exhibit Qst values within the 20–35 kJ mol⁻¹ regime—widely recognized as the optimal thermodynamic “sweet spot” for physisorption. Materials in this window bind CO₂ strongly enough to ensure high uptake at near-ambient pressures while preserving fast adsorption kinetics and low regeneration energies. Representative systems include oak-leaf ACs (23–32 kJ mol⁻¹) (Serafin and Cruz, 2022), fern-leaf ACs (27–32 kJ mol⁻¹) (Serafin et al., 2023a), pine-sawdust carbons (23–33 kJ mol⁻¹) (Patel et al., 2023), and nitrogen-doped porous carbons prepared via solvent-free melt polycondensation (25–30 kJ mol⁻¹) (Xiao et al., 2022). These sorbents—despite their distinct precursor chemistries—display convergent thermodynamic behavior shaped by ultramicropore (<0.7 nm) enrichment and moderate basic functionalities.

Other nitrogen-containing carbons demonstrate slightly weaker interactions. For instance, urea-modified bio-oil carbons (15–22 kJ mol⁻¹) (Li et al., 2024) fall on the lower edge of the physisorption window, consistent with a higher fraction of less-basic nitrogen species. By contrast, sorbents incorporating highly reactive heteroatoms exhibit significantly stronger binding energies. Sulfur-doped carbons from PPS (22–41 kJ mol⁻¹) (Bai et al., 2023) show elevated Qst values arising from oxidized sulfur species that enhance dipole–quadrupole interactions with CO₂. Even more pronounced interactions were reported for crustacean-shell–derived hierarchical carbons (41–49 kJ mol⁻¹) (Shao, 2024), where pyrrolic-N, Ca-based functionalities, and ultramicroporosity act synergistically to strengthen CO₂ binding—approaching the boundary between strong physisorption and partial chemisorption.

A similar dual-regime behavior appears in pine-needle biochar–derived carbons (18–42 kJ mol⁻¹) (Lim et al., 2023), where low-temperature activation retains Ca(OH)₂ and nitrogen functionalities that elevate Qst to >40 kJ mol⁻¹, while higher-temperature activation produces micropore-dominated structures with Qst values in the 30–36 kJ mol⁻¹ range. Mild heteroatom enrichment also influences Qst in lemon-peel–derived carbons, in which KOH + alum activation yields 23 kJ mol⁻¹ whereas pure KOH activation increases Qst to 35 kJ mol⁻¹ (Weldekidan et al., 2023).

Collectively, these results define a robust thermodynamic framework for AC-based CO₂ capture:

Qst < 20 kJ mol⁻¹ → weak physisorption, low affinity.

Qst = 20–35 kJ mol⁻¹ → ideal regime, balancing strong CO₂ binding with facile regenerability (as in Patel et al., 2023; Serafin and Cruz, 2022; Serafin et al., 2023a; Xiao et al., 2022).

Qst > 40 kJ mol⁻¹ → enhanced selectivity but increased regeneration energy; often linked to heteroatom-rich or mineral-assisted sites (as in Bai et al., 2023; Lim et al., 2023; Shao, 2024).

These thermodynamic insights highlight the need for simultaneous optimization of ultramicropore volume, surface basicity, and heteroatom speciation to maintain strong yet reversible CO₂ adsorption. Sorbents engineered to remain within the 20–35 kJ mol⁻¹ Qst —particularly those dominated by ultramicropores and controlled N- or O-functionalities—show the greatest promise for delivering energy-efficient, cyclable, and industrially scalable CO₂ capture performance

Selectivity and competitive adsorption

CO₂/N₂ selectivity is a critical performance indicator for post-combustion CO₂ capture, as flue gas streams typically contain a high proportion of N₂. The compiled data (Figure 4) show substantial variability in selectivity values reported for AC-based sorbents, which can be primarily attributed to differences in precursor type, pore-size distribution, and surface chemistry. Figure 4 compares CO₂/N₂ selectivity values for representative ACs reported between 2020 and 2025 under near-ambient conditions, revealing clear structure–performance trends.

Figure 4.

Reported CO₂/N₂ selectivity values of activated carbons from recent studies (2020–2025) under near-ambient conditions.

Materials engineered with high fractions of ultramicropores (<0.7 nm), in combination with targeted surface functionalities, consistently exhibit enhanced selectivity. Olive-stone-derived ACs (Serafin et al., 2023b) demonstrate an exceptional selectivity of ∼161, significantly outperforming most biomass- and waste-derived carbons, which generally lie within the range of 20–80. In contrast, carbons derived from waste surgical masks (Serafin et al., 2022) and MOF/AC composites (Li et al., 2024) show more moderate selectivity values, emphasizing the strong influence of precursor identity and post-synthetic modification techniques. These observations reinforce the conclusion that achieving superior CO₂/N₂ separation performance requires a synergistic balance of optimized ultramicroporosity and purposeful surface functionalization—particularly heteroatom doping and bio-organic activation routes that introduce basic or polar sites capable of selectively interacting with CO₂.

Overall, the reviewed studies demonstrate that high CO₂/N₂ selectivity arises from the synergistic interplay between ultramicroporosity and appropriately engineered basic surface functionalities. In contrast, materials that depend primarily on high BET surface area—without deliberate control over pore-size distribution or surface chemistry—tend to exhibit only moderate separation factors. These findings underscore the importance of rational pore design and targeted heteroatom doping in tailoring ACs for effective CO₂/N₂ separation under realistic flue-gas conditions.

Stability and regeneration

The cyclic stability of ACs is a critical parameter for their practical deployment in CO₂ capture systems. Across the studies reviewed, most AC-based sorbents retain approximately 90–99% of their initial CO₂ uptake after repeated adsorption–desorption cycles, reflecting the predominance of physisorption-driven interactions, which enable facile and energy-efficient regeneration.

For example, mask-derived AC reported by Serafin et al. (2022) maintained nearly its full uptake capacity after 20 cycles, while oak-leaf-derived carbons (Serafin and Cruz, 2022) preserved ∼99% of their original capacity after 30 cycles. Similarly, waste-tea-derived AC enhanced with SiO₂ nanoparticles (Tahmasebpoor et al., 2023) retained 94.5% of its capacity after 25 cycles and exhibited improved fluidization behavior—an advantageous property for large-scale reactor applications. Nitrogen-doped ACs produced via ammonia treatment or plasma activation also demonstrated strong cyclic stability, maintaining >95% of their CO₂ uptake after 5–10 cycles (Lim et al., 2022), indicating that N-containing functionalities remain stable when the microporous network is preserved. In contrast, sorbents functionalized with amines—such as PEI-modified tire-char carbons—experienced gradual performance deterioration due to pore blockage, amine volatilization, and oxidative degradation. This highlights the inherent trade-off between achieving high initial uptake through chemisorptive interactions and maintaining long-term structural and chemical stability. Carbons exhibiting very high isosteric heats of adsorption, such as crustacean-shell-derived materials with Qst values of 41–49 kJ mol⁻¹ (Shao, 2024), are thermally and chemically robust but require higher regeneration energy. S-doped porous carbons derived from PPS resin exhibited minimal capacity loss after five cycles, demonstrating short-term durability under moderate physisorptive conditions (Bai et al., 2023).

Overall, the evidence indicates that ACs combining abundant ultramicropores with stable heteroatom functionalities—operating primarily through physisorption—offer the most favorable balance of high capacity, low regeneration energy, and long-term cyclic stability. These characteristics underscore their strong potential for industrial post-combustion CO₂ capture, where repeated adsorption–desorption cycling is essential. It should also be stressed that the vast majority of cyclic stability and regeneration tests reported for AC-based sorbents have been performed under simplified gas compositions, often using dry CO₂ or binary CO₂/N₂ mixtures over a relatively small number of cycles. Long-term dynamic operation in the presence of realistic concentrations of SO₂, NOₓ, and water vapor is rarely examined in a systematic manner. Consequently, the true working capacity, deactivation rate, and regeneration penalty of ACs in industrial post-combustion capture units are still poorly quantified. Addressing this knowledge gap will require extended breakthrough and cycling campaigns (up to hundreds of cycles) under fully representative CO₂/N₂/H₂O/SO₂/NOₓ feeds, so that material-level properties can be rigorously linked to process-level performance and lifetime in real flue-gas environments.

Integrated structure–performance map

The performance of ACs in CO₂ capture cannot be attributed to a single parameter; rather, it emerges from the coupled effects of pore structure, surface chemistry, adsorption energetics, and operational stability. The analyses presented in Sections 4.1–4.5 enable the construction of a structure–performance framework that can guide the rational design of advanced sorbents.

First, pore architecture plays a dominant role. Ultramicropores (<0.7 nm) are consistently associated with the highest CO₂ uptakes under near-ambient conditions, whereas total BET surface area alone is an unreliable predictor of performance. This underscores the importance of deliberately engineering the micropore size distribution instead of merely maximizing overall surface area.

Second, surface chemistry provides complementary benefits. Pyridinic and pyrrolic nitrogen functionalities, oxidized sulfur species, and oxygen-containing groups generate favorable acid–base and dipole–quadrupole interactions that significantly enhance CO₂ affinity and CO₂/N₂ selectivity. However, excessive heteroatom doping or heavy amine impregnation can lead to pore blockage and reduced accessibility, highlighting the need to balance chemical modification with the preservation of open porosity.

Third, thermodynamic parameters govern the trade-off between binding strength and regenerability. Most ACs exhibit optimal performance when the isosteric heat of adsorption (Qst) lies in the range of 20–35 kJ mol⁻¹, which affords sufficiently strong CO₂ binding while still enabling energy-efficient desorption. Materials with higher Qst values (>40 kJ mol⁻¹) may offer improved selectivity but typically require greater regeneration energy, potentially compromising process-level energy efficiency.

Finally, cyclic stability integrates these factors under realistic operating conditions. ACs featuring tailored ultramicroporosity and stable heteroatom functionalities, with adsorption dominated by physisorption, routinely exhibit ≥95% capacity retention over multiple cycles. In contrast, heavily functionalized sorbents with very high Qst or high amine loadings can suffer gradual performance degradation due to pore blockage, oxidative side reactions, or active-site loss.

Taken together, these observations define a practical design map for advanced AC-based sorbents:

generate a high fraction of ultramicropores to maximize capacity;

introduce targeted heteroatom functionalities to enhance selectivity;

maintain adsorption energies within the 20–35 kJ mol⁻¹ window to ensure regenerability; and

verify multi-cycle stability under relevant operating conditions.

This unified perspective indicates that the most promising ACs are those that achieve a balanced optimization across all four dimensions—structure, chemistry, energetics, and stability—thereby paving the way for scalable and environmentally sustainable CO₂ capture technologies.

Integrated quantitative comparison of precursor types, activation methods, and performance metrics

To provide a unified evaluation framework and enable horizontal comparison across the 30 representative studies reviewed, the key performance metrics were quantitatively consolidated and examined collectively. Three principal variables—precursor category, activation route, and resulting textural/surface attributes—were compared against common adsorption indicators (CO₂ uptake at 25 °C and 1 bar, CO₂/N₂ selectivity, ultramicropore fraction, and stability).

Biomass-derived precursors activated by KOH yield the highest overall performance, with averaged BET surface areas of 1100–1700 m² g⁻¹, micropore volumes in the range of 0.45–0.70 cm³ g⁻¹, and CO₂ uptakes of 5.0–7.5 mmol g⁻¹. Waste-derived feedstocks activated by KOH show slightly lower textural metrics (BET typically 800–1400 m² g⁻¹), but comparable uptakes (3.5–6.0 mmol g⁻¹) owing to their high ultramicropore fraction. In contrast, CO₂/steam physical activation provides superior sustainability but produces narrower microporosity (0.20–0.45 cm³ g⁻¹) and reduced uptakes of 2.5–4.5 mmol g⁻¹.

A comparison across activation methods shows that KOH activation consistently provides the greatest ultramicropore development and selectivity (up to ∼160), while H₃PO₄ activation yields broader micro–mesopore distributions that favor moderate capacities but improved cyclic stability. Composite formation (AC/MOF or AC/oxide hybrids) enhances selectivity and high-pressure adsorption but does not surpass KOH-ACs under ambient-pressure conditions.

A cross-study analysis also indicates that CO₂ uptake correlates more strongly with ultramicropore volume (R ≈ 0.82) than with BET surface area (R ≈ 0.41), confirming ultramicroporosity as the dominant controlling parameter. Moreover, nitrogen- or sulfur-doping increases selectivity by 20–70% when introduced without significant pore blockage.

This consolidated evaluation framework provides a unified basis for comparing diverse CO₂ sorbents and highlights the precursor–activation combinations that most efficiently produce ultramicroporous structures suited for near-ambient CO₂ capture. It also reveals the trade-offs among activation intensity, surface functionality, and sustainability, thereby offering clearer design guidance than isolated data sets.

Challenges and future perspectives

Despite the significant progress made in developing ACs for CO₂ capture, several challenges remain before these materials can be deployed at industrial scale. These challenges relate to material design, real-world performance, process integration, and scalability.

Cost-Effectiveness and scalability

Although biomass and waste-derived precursors offer inherent environmental benefits, the life-cycle cost and environmental footprint of their conversion into AC remain strongly dependent on the activation route. KOH activation, for example, typically requires 650–850 °C and reagent-to-precursor ratios of 3–5 kg KOH per kg of carbonized biomass, corresponding to an energy demand of approximately 3.2–5.5 MJ kg⁻¹ and generating significant alkaline wastewater streams during post-washing. Only a few pilot-scale studies report partial chemical recovery (40–60% KOH recycling), and fully closed-loop reagent management is rarely demonstrated. These factors indicate that the overall environmental advantage of biomass-derived ACs is highly sensitive to process configuration, reagent recovery efficiency, and the extent of low-temperature or green activation steps used.

Performance under realistic flue gas conditions

Most laboratory studies evaluate CO₂ adsorption using pure gas streams under idealized conditions. However, real flue gas contains multiple impurities—such as H₂O vapor, SO₂, NOₓ, and O₂—that can significantly influence adsorption behavior. Despite the clear practical importance of such multicomponent effects, the quantitative database available for ACs remains surprisingly sparse. Among the 30 representative studies surveyed in this review, only a few report CO₂ adsorption or breakthrough behavior in the presence of defined amounts of SO₂ and NOₓ, and these experiments are typically conducted at one or two fixed impurity concentrations and temperatures. As a result, the existing literature allows only qualitative or semi-quantitative conclusions to be drawn regarding impurity effects: acidic gases are known to strongly interact with basic surface sites, to accelerate the deactivation of N-rich or amine-functionalized carbons, and to lower the effective CO₂ working capacity, whereas more hydrophobic or only mildly basic ACs tend to exhibit better tolerance. However, there are currently insufficient systematic data to derive robust correlations between impurity concentration, temperature, and dynamic CO₂ uptake or selectivity without over-interpreting isolated results. In the present review, we therefore deliberately refrain from proposing empirical fits or design equations and instead emphasize this lack of comprehensive parametric studies as a critical gap that must be addressed before reliable process-level models for industrial flue-gas capture can be established. ACs generally exhibit superior moisture tolerance compared with zeolites or many MOFs, yet their selectivity and long-term stability under multi-component flue gas conditions remain insufficiently characterized. Robust assessment of industrial feasibility therefore requires extended testing under humid, impurity-rich environments to evaluate competitive adsorption effects, pore blocking, oxidative degradation, and cyclic stability under operationally relevant conditions.

Balancing textural and chemical optimization

A recurring challenge in recent research is achieving an optimal balance between textural engineering and chemical functionalization. Excessive heteroatom doping or heavy amine loading can enhance CO₂ affinity but may simultaneously obstruct ultramicropores, reducing accessible adsorption sites and impairing diffusion. Future efforts should adopt integrated design strategies that simultaneously optimize ultramicropore volume while incorporating heteroatom functionalities in a controlled manner. Advanced computational tools—including molecular simulations, density functional theory, and machine-learning-assisted optimization—can accelerate the rational tuning of pore structures and surface chemistries, enabling targeted design of high-performance sorbents without compromising pore accessibility.

Energy-efficient regeneration

Although most ACs operate within a favorable Qst window (20–35 kJ mol⁻¹), sorbents with higher adsorption enthalpies (>40 kJ mol⁻¹) require more energy for regeneration, which can diminish overall process efficiency. The development of “smart” sorbents with tunable binding strengths—either through tailored surface functionalities or controlled pore environments—represents a promising strategy to mitigate regeneration costs. In parallel, the integration of advanced swing adsorption processes (e.g., temperature swing adsorption, vacuum swing adsorption, or pressure swing adsorption) can further reduce energy penalties by aligning regeneration conditions with the thermodynamic characteristics of the sorbent.

Integration with emerging CO₂ capture technologies

Emerging CO₂ capture technologies such as direct air capture (DAC) and modular, small-scale capture units demand sorbents that combine high selectivity with low regeneration energy. ACs—particularly those derived from abundant biomass and waste feedstocks—are strong candidates for these applications, provided that ultramicroporosity and stable heteroatom functionalities can be reliably engineered. Future research should explore hybrid systems, such as AC/MOF and AC/polymer composites, to exploit synergistic effects in adsorptive capacity, selectivity, and processability, thereby broadening the application space from post-combustion capture to DAC and decentralized capture platforms.

Environmental and sustainability considerations

Although the conversion of agricultural residues, plastics, and medical wastes into ACs aligns with circular-economy principles, the environmental footprint of activation agents (e.g., KOH, ZnCl₂, and H₃PO₄) and post-processing steps (particularly washing and neutralization) must be carefully evaluated. Transitioning toward greener activation methods—such as physical activation, bio-derived activating agents, or low-impact chemical routes—and ensuring safe treatment, recycling, or recovery of process by-products will be essential for the long-term sustainability and regulatory acceptance of AC-based CO₂ sorbents.

The future of AC-based CO₂ sorbents will depend on holistic optimization, in which textural properties, surface chemistry, thermodynamic efficiency, and operational stability are improved in a coordinated manner. Realizing this vision will require: (i) valorization of green and waste-derived precursors; (ii) data-driven and simulation-assisted materials design; and (iii) rigorous validation under industrially relevant conditions and process configurations. By integrating these elements, laboratory-scale advances in ACs can be translated into practical, scalable, and environmentally sustainable solutions for CO₂ capture across power, industrial, and emerging DAC applications.

It is also important to acknowledge that comprehensive life-cycle assessments (LCA) and life-cycle cost analyses (LCC) for AC synthesis remain scarce. Most published reports provide either qualitative claims of sustainability or partial data (e.g., activation temperature, reagent consumption, washing steps) that cannot be directly integrated into a unified environmental model. Furthermore, the stability and scalability of precursor supply—such as seasonal agricultural residues or geographically dispersed waste streams—introduce logistical and transportation-related impacts that are seldom quantified. As a result, the long-term environmental advantage of biomass-derived ACs can only be fully validated through systematic cradle-to-gate LCA/LCC studies, which represent a critical research gap for guiding industrial deployment and technical transformation.

Breakthrough performance and practical applicability

Beyond equilibrium isotherms, which are typically measured under idealized conditions, fixed-bed breakthrough experiments provide the most realistic measure of how ACs perform in continuous CO₂ capture processes. Breakthrough curves directly probe mass-transfer resistance, bed utilization, column hydrodynamics, and the influence of gas-phase composition, all of which determine the usable capacity of a sorbent in a real unit rather than its theoretical maximum uptake. In practice, dynamic capacities obtained from breakthrough tests are often substantially lower than equilibrium values, and the shape of the breakthrough front (sharp versus tailing) reveals whether pore diffusion, surface reaction, or external film resistance is rate-limiting.

Recent studies on heteroatom-doped carbons nicely illustrate these points. For example, N,S-codoped coconut-shell ACs showed high equilibrium uptakes (∼4.4 mmol g⁻¹ at 25 °C and 1 bar), but breakthrough tests in a packed bed (10 vol% CO₂ in N₂, 25 °C, 1 bar, 10 mL min⁻¹) yielded a dynamic capacity of only ∼0.97 mmol g⁻¹, i.e., roughly one quarter of the static capacity (Shao et al., 2025). The corresponding breakthrough curve was very steep, with minimal tailing, indicating fast mass transfer and efficient bed utilization, and the sorbent retained >99% of its capacity over at least five cycles, confirming the robustness of physisorption-dominated uptake. A similar behavior was reported for B-doped porous carbons (WSCPM-900-2), where an equilibrium uptake of ∼3.15 mmol g⁻¹ at 25 °C translated into a dynamic breakthrough capacity of ∼0.85 mmol g⁻¹ under 10 vol% CO₂/N₂ at 1 bar (Wang et al., 2025). Again, a sharp breakthrough front and excellent multi-cycle stability demonstrated favorable mass-transfer characteristics and structural durability, while simultaneously highlighting the systematic gap between equilibrium and in-process capacities.

These examples emphasize that, for biomass- and waste-derived ACs, breakthrough testing is indispensable for assessing practical usability. Materials that appear promising based on equilibrium isotherms alone may deliver much lower working capacities once flow, contact time, and multicomponent competitive adsorption are accounted for. Systematic breakthrough measurements under realistic post-combustion conditions (e.g., 10–15% CO₂ in N₂ with water vapor and trace SO₂/NOₓ), combined with multi-cycle testing, should therefore become a standard part of sorbent evaluation protocols. Incorporating such dynamic metrics will be crucial for bridging the current gap between laboratory-scale adsorption data and the design of industrially relevant CO₂ capture processes based on ACs. In addition, almost all breakthrough studies considered to date have been carried out in the absence of SO₂ and NOₓ, which further limits the direct transferability of the reported dynamic capacities to industrial flue-gas conditions and reinforces the need for impurity-resolved breakthrough testing.

Conclusion

Over the past three years (2022–2025), research on ACs for CO₂ capture has expanded rapidly, driven by the use of diverse biomass and waste precursors, innovative activation strategies, and targeted surface functionalization. This review has synthesized findings from 30 representative studies, elucidating clear structure–performance relationships and outlining design principles that can guide the development of next-generation AC-based sorbents.

The analysis leads to three central conclusions:

Ultramicropores (<0.7 nm) constitute the most critical structural feature, as they consistently enable high CO₂ uptake under near-ambient pressures. In contrast, total BET surface area alone is an unreliable predictor of performance.

Surface chemistry, particularly the presence of pyridinic and pyrrolic nitrogen as well as oxidized sulfur groups, substantially enhances CO₂ affinity and CO₂/N₂ selectivity when these functionalities are introduced without compromising accessible microporosity.

Thermodynamic efficiency, characterized by isosteric heats of adsorption (Qst) in the range of 20–35 kJ mol⁻¹, provides an optimal compromise between strong binding and facile regeneration, which underpins the excellent multi-cycle stability (>90–99% capacity retention) observed for many AC sorbents.

Despite these advances, several challenges must be addressed before large-scale implementation is realized. Key issues include the scalability and cost-effectiveness of synthesis routes, performance under realistic flue gas conditions, and further reduction of regeneration energy demands. Addressing these challenges will require integrated strategies that combine sustainable precursor utilization, controlled pore engineering, optimized heteroatom functionalization, and systematic long-term testing under industrially relevant conditions.

ACs—especially those derived from renewable and waste resources—are uniquely positioned to contribute to carbon mitigation efforts. By coupling materials innovation with process-level optimization and integration, AC-based sorbents can transition from promising laboratory prototypes to scalable, sustainable, and economically viable solutions for CO₂ separation in both point-source capture and direct air capture applications.