Rational design of biochar for carbon dioxide capture and catalytic conversion

Pyrolysis

Pyrolysis is the most employed route for biochar production, involving the thermal decomposition of biomass in an oxygen-limited atmosphere at elevated temperatures, typically between 300 and 900 °C, and in some cases up to 1,200 °C [Figure 1A]^[40]. Depending on operational parameters such as heating rate and vapor residence time, pyrolysis can be categorized into slow, fast, and flash modes. Among these, slow pyrolysis is particularly favorable for biochar generation, as it enables efficient volatilization of organic compounds while maximizing solid carbon yield^[41]. For this reason, most biochar synthesis processes are conducted via slow pyrolysis under inert gas environments such as nitrogen (N₂) or argon^[42,43]. The resulting biochar yield and properties are highly sensitive to processing conditions. At relatively lower temperatures (300-500 °C), the produced biochar tends to retain abundant oxygen-containing functional groups, including hydroxyl and carboxyl functionalities. In contrast, increasing the temperature above 700 °C and prolonging the residence time promote aromatic condensation and partial graphitization, thereby enhancing electrical conductivity and pore development while reducing the heteroatom content^[44,45]. Moreover, heating rate and reaction duration significantly affect carbon microstructure and porosity evolution. Slow pyrolysis generally leads to higher carbon yields and more structurally ordered frameworks, whereas fast pyrolysis typically produces defect-rich and less ordered carbon structures^[46].

Figure 1. (A) Graphical illustration of the pyrolysis procedure. (B) Schematic representation of the hydrothermal carbonization process. This figure quoted with permission^[40]. Copyright 2026, Elsevier.

Hydrothermal carbonization

HTC has been widely explored as a complementary approach to conventional pyrolysis for converting biomass into biochar^[40]. This process is carried out in a closed reaction system where water functions simultaneously as solvent and reaction medium under self-generated pressure, typically at moderate temperatures between 180 and 250 °C [Figure 1B]. The solid product formed through HTC, commonly termed hydrochar, exhibits enhanced carbonization compared with the original biomass precursor^[47].

A key advantage of HTC lies in its ability to process wet biomass directly, eliminating the need for pre-drying. This feature makes the technique particularly attractive for treating moisture-rich feedstocks such as food waste, agricultural by-products, and sewage sludge^[48-50]. Throughout the hydrothermal process, biomass undergoes a cascade of chemical transformations, including hydrolysis, dehydration, and decarboxylation, which collectively modify its molecular structure and carbon framework. Compared with high-temperature pyrolysis-derived biochar, hydrochar generally retains a higher proportion of oxygen-containing functionalities and exhibits a distinct surface chemistry^[51]. The final material characteristics can be systematically tuned by adjusting reaction temperature and duration. Lower temperatures tend to preserve surface functional groups, which are advantageous for chemical modification and adsorption-related applications^[52]. Conversely, increasing reaction severity promotes aromatic condensation and structural ordering, thereby enhancing electrical conductivity. Owing to this controllability, HTC-derived carbons are considered promising candidates for applications requiring abundant surface functionalities, such as CO₂ capture^[53,54] and catalyst-support engineering^[55].

Gasification

Gasification is a high-temperature thermochemical process that converts carbonaceous feedstocks into a mixture of gaseous products under limited oxygen conditions, typically at temperatures between approximately 700 and 1,400 °C [Figure 2A]^[56]. During this process, biomass undergoes a sequence of transformations including drying, thermal decomposition, partial oxidation, and reduction reactions. The main products are combustible gases such as hydrogen (H₂), carbon monoxide (CO), CO₂, and methane (CH₄), along with minor amounts of N₂ depending on the gasifying agent. In addition, condensable hydrocarbons and solid carbonaceous residue are formed^[57]. Unlike pyrolysis, which is primarily designed to maximize solid carbon yield, gasification aims to enhance syngas production for energy applications. This objective is achieved by introducing controlled amounts of oxidizing agents such as oxygen, air, or steam at elevated temperatures^[56,58]. As a result, only a limited fraction of carbon remains in the solid phase, leading to significantly lower biochar yields compared with pyrolysis^[59]. Furthermore, the residual char obtained from gasification generally exhibits less developed porosity, reduced specific surface area, and diminished surface functional group density relative to biochar produced via conventional pyrolysis^[60,61]. The combination of low solid yield, limited control over surface functionality, and difficulty in tailoring active-site distribution makes gasification less suitable as a dedicated route for high-performance biochar production.

Figure 2. (A) Schematic representation of gasification. Reprinted with permission^[56]. Licensed under CC BY 4.0. (B) Overview schematic of microwave-assisted pyrolysis. Reprinted with permission^[62]. Copyright 2022, Elsevier.

Microwave-assisted pyrolysis

MAP represents a microwave-driven thermochemical approach for biomass conversion [Figure 2B]^[62]. Unlike conventional furnace heating, microwave irradiation delivers electromagnetic energy directly into the material, enabling volumetric heating throughout the biomass matrix. This mechanism allows for rapid temperature rise and more uniform internal heating, often resulting in improved energy utilization efficiency compared with traditional externally heated pyrolysis systems^[63]. The distinct heating characteristics of MAP can significantly influence the structure of the resulting biochar. Accelerated heating rates and efficient energy transfer may promote enhanced pore development and increased specific surface area. Moreover, localized high-temperature zones generated by microwave irradiation can intensify biomass decomposition and facilitate structural reorganization of carbon frameworks, potentially improving adsorption-related properties. Owing to these advantages, microwave-assisted pyrolysis is generally considered more energy-efficient than conventional electrically heated pyrolysis and has been widely explored for applications such as catalysis and soil remediation^[64,65]. However, the performance of MAP-derived biochar is highly dependent on operational parameters such as microwave power, irradiation time, and frequency. Excessive localized heating may lead to thermal runaway or superheating effects, which can cause pore collapse and structural degradation. Therefore, precise control of microwave input and careful optimization of process conditions are essential to ensure uniform heating and preserve the structural integrity of the produced biochar.

Activation strategies

Biochar produced solely through thermochemical conversion typically exhibits lower specific surface area and less-developed porosity compared with commercial activated carbons^[66]. To overcome these inherent limitations, additional activation treatments are commonly applied to tailor its structural and surface properties. Activation strategies are broadly categorized into two main types: physical activation and chemical activation^[67]. While physical activation relies on controlled gas-solid reactions at high temperatures to develop porosity, chemical activation involves the use of activating agents that promote simultaneous carbonization and pore formation^[68-70]. Through these activation processes, pore architecture can be significantly enhanced, surface functional groups can be introduced, and defect density can be precisely tuned^[71,72]. Such structural and chemical modifications are crucial for optimizing biochar performance in carbon capture and broader CCU applications.

Pre-combustion & post-combustion CO₂ capture technologies

Pre-combustion CO₂ capture is primarily implemented in Integrated Gasification Combined Cycle (IGCC) systems. As shown in Figure 3, the process begins with the conversion of fossil fuel feedstocks such as coal, natural gas, or biomass into syngas, which mainly consists of CO and H₂^[87]. This conversion step is referred to as gasification when solid fuels are employed, whereas partial oxidation is used for gaseous or liquid feedstocks. Under high-temperature and high-pressure conditions, the feedstock reacts with controlled amounts of oxidants and steam to produce syngas. The resulting syngas subsequently undergoes catalytic conversion through the water-gas shift reaction, which converts CO into CO₂ while maximizing H₂ production. A key advantage of this pathway is its operation under elevated pressure, which significantly increases the partial pressure of CO₂ and thereby facilitates its separation and removal from the gas stream^[88]. After separation, CO₂ is compressed and dehydrated for transport and storage, while the purified H₂ can be utilized as a carbon-free fuel in gas turbines or fuel cells.

Figure 3. Schematic depiction of pre-combustion carbon capture process. Reprinted with permission^[87], licensed under CC BY 4.0.

From a techno-economic perspective, pre-combustion carbon capture offers distinct advantages, including lower H₂O consumption compared to other capture methods and the co-production of valuable H₂ fuel^[89]. Despite the favorable thermodynamic conditions for CO₂ separation, the overall capital cost of IGCC plants remains high. Moreover, the complexity associated with integrating pre-combustion carbon capture into existing power plants limits its applicability to new installations, often companied by challenges related to reduced overall plant performance.

Post-combustion CO₂ capture involves the separation of CO₂ from flue gas generated after the complete combustion of carbon-based fuels^[90]. This approach represents the most mature and widely implemented capture strategy, largely due to its downstream configuration, which allows retrofitting into existing power plants without significant modification of the combustion process^[91]. However, prior to CO₂ separation, the flue gas typically requires pretreatment processes, such as desulphurization and denitrification, to eliminate impurities that can adversely affect capture performance^[92]. A variety of post-combustion capture technologies have been developed, including chemical absorption, physical adsorption, membrane separation, and cryogenic distillation^[93]. Among these, chemical absorption using amine-based solvents is the most commercially established technology^[94,95]. Despite its high capture efficiency, this approach suffers from several drawbacks, including substantial energy penalties associated with solvent regeneration, corrosion-related operational issues, and the formation of potentially harmful degradation products. Alternative approaches also face inherent limitations. Cryogenic distillation can produce high-purity liquid CO₂ suitable for direct transport, but its application is often restricted by the large energy demand required for refrigeration^[96]. Although membrane separation is attractive due to its chemical-free operation and simplicity, its efficacy is limited by the low CO₂ partial pressure in post-combustion flue streams as well as the susceptibility of membrane degradation from H₂O vapor.

Consequently, solid-based adsorption has emerged as a promising research direction due to its potential for improved energy efficiency and reversible operation via temperature or pressure swing cycles^[12,97]. A wide range of adsorbent materials such as zeolites, biochar, and metal-organic frameworks are actively being investigated. Nevertheless, the industrial scalability of adsorption-based capture remains constrained by trade-offs between adsorption capacity, selectivity, regeneration energy, and long-term stability under realistic flue gas conditions. Exposure to impurities like SO_x and NO_x can lead to adsorbent deactivation, posing significant challenges for sustained operation^[98,99]. Therefore, bridging the gap between laboratory-scale performance and industrial application remains a critical hurdle for adsorption technologies, especially with respect to durability and efficient gas-solid contact under practical operating conditions.

CO₂ capture strategies using biochar-based adsorbents

Biochar, a carbon-rich solid produced through the thermal decomposition of biomass under inert conditions, has emerged as a promising and sustainable precursor for carbon capture applications. One of the primary advantages of biochar is its economic viability. In contrast to commercial activated carbon, biochar production generally requires less energy-intensive activation processes, making it a cost-effective option for large-scale deployment^[100]. Despite these advantages, the direct utilization of pristine biochar is often constrained by its inherent material limitations. In its untreated form, biochar frequently exhibits a relatively low specific surface area and insufficient surface alkalinity, which hinders it from achieving the high adsorption capacities required for industrial implementation. As a result, improving the performance of biochar necessitates a fundamental understanding of the two primary adsorption mechanisms involved in carbon capture, namely physisorption and chemisorption.

Physisorption is predominantly governed by the textural properties of the material, including micropore volume and overall surface area, which facilitate the physical confinement of CO₂ molecules^[101]. In contrast, chemisorption relies on surface chemical interactions between acidic CO₂ molecules and basic active sites^[102]. These sites are primarily associated with functional groups, such as nitrogen-containing species, or with intrinsic inorganic components like alkaline earth metals.

To bridge the performance gap between pristine biochar and commercial adsorbents, recent studies have focused on various modification strategies that regulate both pore architecture and surface chemistry. Although physisorption and chemisorption are discussed separately for mechanistic clarity, they generally coexist in engineered biochars because activation and functionalization can simultaneously alter pore structure, defect density, surface polarity, and active-site accessibility. Their relative contributions therefore cannot be determined from adsorption capacity alone.

Complementary characterization methods are required for reliable mechanism assignment. Low-temperature CO₂ desorption, moderate adsorption heat, and reversible uptake generally support weak pore filling, whereas desorption extending to higher temperatures and the in situ Fourier transform infrared spectroscopy (FTIR) identification of carbonate, bicarbonate, carbamate, or other surface-bound species provide stronger evidence of chemical binding^[103]. Humidity-dependent measurements can reveal competitive H₂O adsorption within micropores and polar sites or, when supported by spectroscopic evidence, moisture-assisted chemical binding. Kinetic models are useful for describing adsorption rates but should not be regarded as independent proof of chemisorption. Accordingly, Sections “Enhancing CO₂ physisorption on biochar through structural optimization” and “Promoting CO₂ chemisorption on biochar via chemical modification” describe the dominant contributions of pore filling and surface interactions, respectively, while Section “Synergistic pore construction and heteroatom functionalization” discusses their synergistic integration for high-performance CO₂ capture.

Enhancing CO₂ physisorption on biochar through structural optimization

The effectiveness of CO₂ physisorption on biochar is primarily governed by the micropore filling mechanism^[104]. Because physical adsorption occurs through the confinement of gas molecules within the porous framework, the development of accessible microporous structures is the key determinant of adsorption capacity. Although increasing the specific surface area generally provides more adsorption sites, it is widely accepted that the availability of well-developed micropores is more critical for efficient CO₂ capture than the total surface area alone^[105].

Pore size distribution also plays a decisive role in determining CO₂ adsorption performance. Micropores in the range of approximately 0.3 to 0.8 nm are considered optimal, with ultra-micropores smaller than 0.5 nm exhibiting the strongest adsorption affinity. Previous studies have further demonstrated that equilibrium CO₂ uptake correlates more strongly with micropore volume than with the total Brunauer-Emmett-Teller surface area, indicating that accessible micropores are more critical than the overall surface area for effective CO₂ capture^[104]. Pores with dimensions comparable to the kinetic diameter of CO₂ (~0.33nm) provide the most favorable adsorption environment, where overlapping adsorption potentials from opposing pore walls significantly enhance van der Waals interactions and CO₂ affinity^[106,107]. In contrast, mesopores and macropores contribute less directly to the equilibrium adsorption capacity but play indispensable roles in facilitating gas diffusion and improving adsorption kinetics by providing efficient transport pathways to the adsorption sites. Therefore, the rational design of biochar should maximize accessible ultramicropore volume while maintaining hierarchical pore connectivity to simultaneously achieve high equilibrium CO₂ uptake and efficient mass transport. Consequently, an effective biochar adsorbent integrates macropores for gas transport, mesopores for efficient diffusion, and micropores as the primary CO₂ adsorption domains^[108].

To validate these structural design principles experimentally, recent studies have adopted synergistic activation strategies to simultaneously optimize micropore development and pore accessibility. Table 2 summarizes previous studies on biochar-based adsorbents for CO₂ capture. Wang et al.^[109] showed the integration of H₂O₂ pre-oxidation with KOH activation enables the fabrication of ultra-microporous biochar from pine sawdust. In this process, mild pre-oxidation introduced oxygen-containing functional groups into the precursor, which significantly enhanced the reactivity of the carbon matrix towards the subsequent chemical etching. As a result, an exceptionally well-developed microporous network was obtained, yielding a remarkable specific surface area of 3,380.4 m²·g^-1 and a microporosity ratio as high as 91.6%. Consistent with the micropore filling mechanism discussed above, this textural optimization led to high CO₂ adsorption capacity of 6.60 mmol·g^-1 at 30 °C, even in the absence of heteroatom doping. These findings confirmed that maximizing narrow microporosity while preserving pore accessibility remains a powerful strategy for CO₂ capture performance dominated by physisorption.

Table 2

Summary of biochar-based adsorbents for CO₂ capture

Primary modification strategies	Adsorbents	Specific surface area (m2 g-1)	CO2 adsorption capacity (mmol·g−1)	CO2 adsorption measurement conditions	CO2/N2 selectivity (15 vol % CO2 in N2)	Ref.
Porous modification	ABC-ahp-6	3,380.4	6.60	Pure CO2/30 °C/1 bar	-	[109]
	RHC-PC-2	1,343.8	1.55	Pure CO2/30 °C/1 bar	37.80	[110]
Non-metallic heteroatom functionalization	AF-700	1,371	3.34	Pure CO2/0 °C/1 bar	72	[126]
	ABC800	836.5	4.06	Pure CO2/0 °C/1 bar	-	[127]
	OPPC31	1,445.71	4.31	Pure CO2/25 °C/1 bar	17	[128]
	ASBC-800-0.5	2,890	2.95	Pure CO2/30 °C/1 bar	44.03	[129]
Introduction of alkali earth metals	5 wt% Mg-loaded biochar	292	1.86	Pure CO2/25 °C/1 bar	-	[134]
	MgO/biochar-1	1,453	6.65 3.72	Pure CO2/0 °C/1 bar Pure CO2/25 °C/1 bar	96.7	[135]
	BC(Ca)	445	2.42	Pure CO2/25 °C/1 bar	36	[136]
Synergistic pore and surface modification	PAC 2	755	3.13 1.55	Pure CO2/0 °C/1 bar Pure CO2/50 °C/1 bar	37	[138]

Benchmark condition: Pure CO₂ at 25 °C and 1 bar under equilibrium adsorption. Values measured under other temperatures, pressures, gas compositions, or dynamic conditions should be interpreted within their respective experimental conditions and not directly compared.

Beyond maximizing micropore volume through conventional chemical activation, recent studies have increasingly explored sustainable synthesis strategies that exploit the intrinsic mineral components of biomass to engineer hierarchical porosity. For example, Gan et al.^[110] developed a green synthesis strategy based on silicon-rich rice husk and potassium citrate. In this approach, naturally embedded silica nanoparticles within the biomass functioned as in-situ hard templates, while potassium citrate served as a low-corrosive activating agent. This dual role prevented structural collapse during carbonization, resulting in the formation of unique nanocapsule-like architectures featuring interconnected transport channels. Subsequent removal of the silica template, together with etching by in-situ generated activating species, created a refined pore network dominated by narrowly distributed ultra-micropores centered at 0.36 and 0.59 nm. As depicted in Figure 4A and B, this optimized biochar (RHC-PC-2) achieved a high CO₂ uptake of 1.55 mmol·g^-1 and a high CO₂/N₂ selectivity of 37.8, markedly outperforming samples prepared by conventional KOH activation. These results emphasized that leveraging natural structural templates offers a viable pathway to fabricate selective adsorbents with interconnected transport pathways, enabling a favorable balance between adsorption performance and environmental sustainability.

Figure 4. (A) Thermogravimetric analysis curves of the synthesized materials. (B) Comparison of CO₂/N₂ selectivity across the as-prepared samples. This figure quoted with permission^[110]. Copyright 2022, Elsevier.

Promoting CO₂ chemisorption on biochar via chemical modification

Beyond pore confinement, surface chemical properties provide complementary contributions to CO₂ adsorption by introducing basic and polar interaction sites. This mechanism is largely driven by Lewis acid-base interactions, in which acidic CO₂ molecules interact with basic active sites on the biochar surface^[111]. Consequently, enhancing surface basicity through the introduction of nitrogen or oxygen containing functional groups, or through the incorporation of alkaline earth metals, is considered an effective strategy for increasing CO₂ capture capacity. In addition to surface basicity, surface polarity also plays a crucial role in adsorption selectivity^[112]. Beyond heteroatom- and metal-derived sites, topological and edge defects within the carbon skeleton can also contribute to CO₂ capture. Non-hexagonal motifs, particularly five-membered rings, together with vacancies and exposed edge sites, disrupt the electronic symmetry of the sp² carbon lattice and induce localized charge redistribution. The resulting electron-rich carbon centers may act as weak Lewis-basic sites toward the electrophilic carbon atom of CO₂, thereby strengthening adsorption and promoting molecular polarization. Depending on the defect configuration and binding strength, this contribution may range from enhanced physisorption to weak chemisorption. Therefore, defect-derived adsorption should be distinguished from micropore confinement and heteroatom effects through complementary Raman, X-ray photoelectron spectroscopy (XPS), CO₂-temperature-programmed desorption (CO₂-TPD), and theoretical analyses rather than inferred from defect density alone^[113]. Moreover, a reduction in hydrophilicity increases surface hydrophobicity, which suppresses the competitive adsorption of H₂O molecules and thereby promotes preferential CO₂ uptake under humid conditions. Most adsorption capacities, however, are reported under dry conditions and may therefore overestimate performance in practical flue-gas streams. Under humid conditions, H₂O competes with CO₂ for polar surface sites and narrow micropores, reducing the accessible adsorption capacity. Although increasing surface hydrophobicity can mitigate this effect, excessive removal of polar functional groups may simultaneously weaken CO₂ affinity. Furthermore, co-adsorbed water increases the energy required for adsorbent regeneration^[114]. Therefore, humid working capacity and long-term cyclic stability under simulated flue-gas conditions should be considered together with dry equilibrium adsorption capacity.

To introduce chemically defined heteroatom- and metal-derived adsorption sites, pristine biochar typically requires chemical modification. Such treatments generally follow two main approaches. In pre-modification strategies, raw biomass is impregnated with chemical agents prior to pyrolysis^[115]. In contrast, post-modification strategies involve chemical treatment and subsequent thermal activation of the biochar after carbonization^[116]. Both approaches aim to enrich the carbon framework with chemically active sites capable of forming strong intermolecular interactions with CO₂, including hydrogen bonding and dipole-related interactions.

Common chemical modifications include oxidation using basic solutions to introduce oxygen-containing functional groups such as carboxyl, phenolic and hydroxyl as well as high temperature amination to incorporate nitrogen-containing functionalities such as amine groups^[117-119]. Additionally, impregnation with metal oxide precursors is frequently utilized to anchor basic metal species onto the biochar matrix. Based on these considerations, the following subsections will systematically categorize and critically analyze chemisorption enhancement strategies focusing on non-metallic functional group modification and metal incorporation.

Non-metallic heteroatoms functionalities

Beyond porous structure engineering, the intrinsic surface chemistry of biochar fundamentally dictates its chemisorption behavior. While the incorporation of metal species creates localized basic sites, the introduction of non-metallic heteroatoms, particularly nitrogen, oxygen and sulfur, serves as a primary strategy for modulating the electronic structure and surface polarity of the carbon matrix^[120-122]. Among these heteroatoms, nitrogen doping has been extensively explored due to its ability to generate electron-rich Lewis basic sites, such as pyridinic, pyrrolic, and amine functionalities, directly within the carbon lattice^[123]. Among these nitrogen species, pyridinic N is generally regarded as the primary active site for CO₂ chemisorption because its lone-pair electrons readily interact with the electrophilic carbon atom of CO₂ through Lewis acid-base interactions. In contrast, pyrrolic N and amine functionalities mainly contribute by enhancing the local electron density and surface basicity of the carbon framework, thereby promoting CO₂ adsorption^[124,125]. Similarly, oxygen-containing ketone and carbonyl groups enhance the surface polarity and strengthen dipole-quadrupole interactions with CO₂, whereas sulfur containing functionalities primarily improve CO₂ affinity by redistributing the electron density and polarizability of adjacent carbon atoms. Collectively, the targeted incorporation of these non-metallic heteroatoms transforms the relatively inert carbon matrix into a chemically active CO₂ adsorption surface, indicating that the capture performance is governed not only by the overall heteroatom content but also by the chemical state and distribution of these surface functional groups.

Building upon these principles, nitrogen doping has been widely adopted as an effective strategy to introduce basic active sites into the carbon lattice, thereby markedly enhancing CO₂ affinity. For instance, Sun et al.^[126] reported the synthesis of nitrogen self-doped hierarchical porous biochar derived from carp fish scales via a KOH-mediated activation process. XPS analysis of the N 1s spectrum of the optimized sample (AF-700) revealed the successful substitution of carbon atoms with nitrogen, predominantly in the form of pyrrolic N (N-5) and pyridinic N (N-6) functionalities. These nitrogen configurations functioned as alkaline active sites that not only strengthened hydrogen bonding but also facilitated electron transfer with weakly acidic CO₂ molecules, creating a highly favorable environment for chemisorption. Benefiting from the synergistic combination of nitrogen-induced chemical interactions and a substantial specific surface area of 1,371 m²·g^-1, the AF-700 biochar achieved a CO₂ uptake capacity of 3.34 mmol·g^-1 at 0 °C and 1 bar. These results provided compelling evidence that utilizing the intrinsic nitrogen of biomass for self-doping can maximize acid-base interactions, thereby driving highly efficient CO₂ capture beyond the limitations of purely physisorption-driven mechanisms.

Active amination processes such as ammonialization represent a highly effective strategy to chemically graft strongly basic functional groups onto the carbon skeleton. For example, Hsu et al. demonstrated a single-step pyrolytic synthesis combining carbonization and ammonialization using Leucaena leucocephala biomass^[127]. During the high-temperature treatment, ammonia gas decomposed into reactive nitrogen radicals (NH^* and NH₂^*), which covalently bonded with the biochar matrix to form abundant amino (C-NH₂) and imine/pyrrolic (C-NH) functional groups [Figure 5A]. These introduced nitrogen species served as potent Lewis basic sites that chemically anchored the weakly acidic CO₂ molecules. The additional behavior was well described by the pseudo-second-order kinetic and Langmiur-Freundlich models, suggesting heterogeneous uptake involving both pore filling and interactions with nitrogen-containing sites. However, kinetic and isotherm fitting alone cannot conclusively establish chemisorption. The optimized ammonialized biochar (ABC800) exhibited a superior CO₂ capacity of 4.06 mmol·g^-1 at 0 °C, indicating that nitrogen functionalization enhanced CO₂ affinity in combination with its ultramicroporous structure.

Figure 5. (A) Possible CO₂ capture mechanisms by chemisorption. Reprinted with permission^[127]. Copyright 2024, Springer. (B) Scheme of the synthesis process for N, S co-doped biochar. Reprinted with permission^[129]. Copyright 2025, Elsevier.

In addition to nitrogen functionalities, the introduction of oxygen-containing functional groups offers another effective avenue to manipulate the surface polarity of biochar, thereby promoting CO₂ adsorption through strong electrostatic interactions. In a recent study, Wang et al.^[128] developed a green synthesis route based on a dual organic salt activation process to produce highly oxygenated porous biochar from orange peels. This approach enabled simultaneous control over porous architecture and surface chemistry, leading to a substantial enrichment of oxygen functionalities on the carbon surface. By integrating experimental characterizations with density functional theory (DFT) calculations, they elucidated the specific roles of distinct oxygen-containing groups in CO₂ adsorption. Notably, ketone functionalities, identified as the dominant oxygen species in the optimized sample, exhibited particularly strong electrostatic interactions with CO₂ molecules. DFT calculations demonstrated that the incorporation of ketone groups dramatically increased the CO₂ adsorption energy by 109.9% compared to a pristine carbon substrate, highlighting their critical contribution to adsorption affinity. Driven by the synergistic effects of a well-developed microporous framework and enhanced surface polarity, the resulting biochar delivered an outstanding CO₂ capture capacity of 4.31 mmol·g^-1 at 25 °C. This work suggested that rational oxygen functionalization can significantly amplify electrostatic interactions, offering an effective chemisorption pathway that complements physical confinement within micropores.

To further push the boundaries of chemisorption, recent advancements have focused on synergistic effects of multi-heteroatom co-doping, particularly combining nitrogen and sulfur. As depicted in Figure 5B, Chen et al.^[129] synthesized an N, S co-doped biochar via microwave pyrolysis, utilizing Enteromorpha as a natural nitrogen precursor and sodium thiosulfate as an external sulfur source. While physical microporosity initiated the capture via van der Waals forces, the remarkable enhancement in CO₂ uptake (reaching 2.95 mmol·g^-1) was primarily driven by the sophisticated surface chemistry. The integration of pyridinic and pyrrolic nitrogen provided direct Lewis basic sites, whereas the incorporation of oxidized sulfur groups fundamentally modulated the electron density and polarizability of the surrounding carbon lattice. XPS and kinetic analyses demonstrated that the dual-doping strategy significantly enhanced acid-base interactions and facilitated rapid electron transfer between the active sites and CO₂ molecules. This synergistic interplay confirmed that strategically coupling distinct heteroatoms not only optimizes physical confinement but fundamentally establishes a highly reactive chemical environment for superior CO₂ chemisorption.

Incorporation of alkaline earth metals

Biomass exhibits an intrinsically heterogeneous elemental composition that depends on its feedstock origin. In addition to carbon, oxygen and nitrogen, biomass typically contains trace amounts of inorganic mineral species, including alkaline earth metals. The inherent presence or intentional enrichment of these metals, particularly calcium (Ca) and magnesium (Mg), plays a critical role in tuning the surface chemistry of biochar^[130-133]. By increasing surface alkalinity, these metal species enhance the affinity of the adsorbent toward acidic CO₂ molecules. This interaction promotes chemisorption mechanisms, enabling CO₂ fixation through the formation of stable carbonate and bicarbonate species. As a result, exploiting these unique chemical properties to improve adsorption capacity has become a major focus in recent biochar-based CO₂ capture research.

To enhance chemisorption performance, the incorporation of alkaline earth metals into biochar has been extensively investigated. Lahijani et al.^[134] studied metal-impregnated biochar derived from walnut shells, highlighting the pivotal role of metal selection in CO₂ adsorption behavior. As illustrated in Figure 6A, among various samples loaded with 5 wt% metals, Mg-modified biochar exhibited the highest CO₂ uptake compared to other counterparts. This superior performance was attributed to the formation of basic magnesium oxide (MgO) species on the carbon surface, which strongly interact with acidic CO₂ molecules. Metal loading was also found to critically influence adsorption capacity. As shown in Figure 6B, increasing Mg content initially enhanced CO₂ uptake due to the increased density of basic active sites. However, excessive metal loading resulted in a decline in performance, likely caused by metal agglomeration and partial blockage of micropores that restrict gas diffusion. These results highlighted the importance of optimizing metal content to balance chemical activity and pore accessibility. In addition to capacity, Mg-loaded biochar revealed high selectivity for CO₂ over N₂ and CH₄ [Figure 6C]. This selectivity arose from preferential acid-base interactions, confirming the potential of Mg impregnated biochar for post-combustion CO₂ capture.

Figure 6. (A) CO₂ adsorption curves for biochars modified with 5 wt% of different metals. (B) CO₂ uptake profiles of biochars for varying Mg loadings. (C) Gas adsorption selectivity of Mg-loaded biochar for CO₂ over competing gases. (A-C) Reprinted with permission^[134]. Copyright 2018, Elsevier. (D) Illustration of the fabrication route for the hierarchically porous MgO/biochar composites. (E) CO₂-TPD profiles of the fabricated samples. (D and E) Reprinted with permission^[135]. Copyright 2024, Elsevier.

To overcome the limitations of conventional metal impregnation and maximize the synergy between physisorption and chemisorption, Li et al.^[135] employed an in situ structural engineering approach, fabricating hierarchically porous MgO/biochar composites via a microwave-assisted hydrothermal method [Figure 6D]. Unlike conventional pyrolysis, this approach facilitated rapid and uniform nucleation of MgO nanocrystals within the carbon matrix. The resulting composite achieved a remarkable specific surface area of 1,453 m²·g^-1 with abundant narrow micropores, providing a favorable architecture for CO₂ adsorption. CO₂-TPD analysis revealed broad desorption features extending toward higher temperatures, consistent with a distribution of MgO-derived basic sites and stronger CO₂ binding than that of pristine biochar [Figure 6E]. These results support a contribution from surface chemical interactions in addition to micropore filling. However, CO₂-TPD alone cannot identify magnesium carbonate or other specific adsorbed species, which requires direct spectroscopic evidence such as in situ FTIR. Consequently, the composite demonstrates how hierarchical porosity and MgO-derived basic sites can jointly contribute to CO₂ capture through improved transport and enhanced adsorption affinity.

Expanding the range of alkaline earth metals, Ca has emerged as a cost-effective alternative for enhancing surface basicity in biochar-based adsorbents. Arjona-Jaime et al.^[136] displayed that Ca-modified biochar prepared through a rapid microwave-assisted irradiation method enabled uniform dispersion of Ca-based active sites within the carbon matrix while avoiding the prolonged processing times associated with conventional thermal treatments. The resulting adsorbent exhibited enhanced CO₂ uptake capacity along with high selectivity toward CO₂ over N₂, which is a key requirement for post-combustion capture applications. Crucially, the Ca-functionalized biochar effectively addressed a major limitation of many physical adsorbents, namely performance degradation under humid conditions. When evaluated under simulated flue gas environments containing H₂O vapor, the adsorbent maintained stable CO₂ uptake. This moisture tolerance suggests that Ca-derived basic sites preserved CO₂ affinity in the presence of H₂O and may have promoted the formation of carbonate and bicarbonate species. Nevertheless, humidity resistance alone cannot conclusively distinguish chemisorption from suppressed competitive H₂O adsorption, and direct spectroscopic identification of the adsorbed species is required. Taken together, these findings suggest that Ca incorporation can increase CO₂ adsorption through acid-base interactions while improving the potential applicability of biochar under humid industrial conditions.

Thermocatalytic CO₂ conversion

Thermocatalytic CO₂ conversion refers to the transformation of carbon dioxide into value-added chemicals and fuels through heat-driven catalytic reactions^[140]. Representative thermocatalytic pathways mainly involve CO₂ hydrogenation reactions, including the reverse water gas shift (RWGS) reaction for CO production, methanation to CH₄, and methanol (CH₃OH) synthesis^[141].

For instance, Pan et al.^[142] developed a Cu-Mo₂C cluster catalyst supported on nitrogen-doped biochar (NBC) derived from Enteromorpha prolifera (Cu-Mo₂C/EPBC) for the RWGS reaction. The algae-derived precursor enabled the formation of N-doped mesoporous carbon enriched with nitrogen functionalities, strengthening interactions with β-Mo₂C and promoting the generation of well-defined Cu-Mo₂C interfacial active sites. Compared with the activated carbon-supported counterpart (Cu-Mo₂C/AC), the EPBC-supported catalyst exhibited higher graphitization, larger surface area, and abundant pyridinic-N species capable of stabilizing Cu-Mo₂C clusters. Catalytic performance was evaluated using a fixed-bed reactor. The catalyst containing 20 wt% Mo₂C and 0.8 wt% Cu achieved optimal CO₂ conversion and CO selectivity. At 500 °C, Cu-Mo₂C/EPBC delivered a CO selectivity of 99.95% and nearly threefold higher CO₂ conversion than Cu-Mo₂C/AC. Control experiments confirmed that the enhanced performance originated from the synergistic effect between pyridinic-N anchoring and Cu-Mo₂C cluster interactions. The catalyst also maintained over 99% of its initial activity after 50 h, demonstrating excellent stability. In the case of CH₃OH synthesis, Xu et al.^[143] reported CuZn catalysts supported on coconut shell-derived mesoporous biochar (CuZn/MC). Reference catalysts were prepared via sequential impregnation, including Cu-Zn/MC, Zn-Cu/MC, and CuZn/SiO₂. The mesoporous carbon framework facilitated uniform dispersion of Cu-ZnO species and promoted the formation of low-valence Cu (Cu⁰/Cu⁺) sites essential for CH₃OH production. One-step incorporation of Cu and Zn strengthened metal-support interactions and increased the abundance of Cu⁺ species. As a result, CuZn/MC exhibited significantly higher CO₂ conversion and space-time yield (STY) than CuZn/SiO₂, while maintaining comparable CH₃OH selectivity [Figure 7A]. Notably, CuZn/MC reached a CH₃OH STY of 121.25 g_MeOH·kg_cat^-1·h^-1 at 260 °C, nearly three times that of CuZn/SiO₂, with stable performance over 50 h [Figure 7B and C]. Similarly, Vali et al.^[144] demonstrated that biochar derived from renewable waste can function as an effective support for Cu/ZnO nanoparticles in CO₂ hydrogenation to CH₃OH. The resulting catalyst delivered a CH₃OH STY of 496.5 mg_MeOH·g_Cu^-1·h^-1 with 71% selectivity and maintained stable operation for more than 45 h. Under identical reaction conditions, its performance markedly surpassed that of conventional Cu/ZnO/Al₂O₃, which exhibited lower productivity and more rapid deactivation. Structural characterization revealed that the enhanced activity and durability originated from enriched Cu-Zn interfacial sites, enabled by the uniform dispersion of ultrasmall nanoparticles and improved H₂ and CO₂ adsorption on the biochar support.

Figure 7. Performance of CO₂ hydrogenation to methanol over (A) series of Cu-Zn catalysts and (B) varying reaction temperatures. (C) Durability evaluation of the CuZn/MC catalyst. (A-C) Reprinted with permission^[143]. Copyright 2025, Elsevier. Influence of pyrolysis temperature on catalytic performance in CO₂ methanation at 400 °C: (D) CO₂ conversion, (E) CH₄ yield. (F) CH₄ yield in CO₂ methanation at 400 °C as a function of preparation method. (D-F) Reprinted with permission^[147]. Copyright 2026, Elsevier.

Beyond CH₃OH synthesis, CO₂ methanation to CH₄ has received considerable attention as an attractive pathway for synthetic natural gas production. Villardon et al.^[145] investigated olive stone-derived biochar as a sustainable support for Ni-based methanation catalysts. To tailor the structural and surface characteristics of the support, chemical activation was performed using KOH, ZnCl₂, or H₃PO₄. Among the resulting materials, the KOH-activated biochar exhibited the highest surface area, superior Ni dispersion, and a greater density of moderately strong basic sites favorable for CO₂ activation. Consequently, this catalyst achieved 72% CO₂ conversion with 95.5% CH₄ selectivity, outperforming the other activated counterparts. In contrast, H₃PO₄ activation led to partial encapsulation of Ni species, while ZnCl treatment promoted Ni-Zn alloy formation, both of which negatively affected methanation performance.

Furthermore, Wang et al.^[146] developed NBC from Pinus sylvestris through an in-situ pyrolysis process employing urea as a nitrogen precursor and NaHCO₃ as an activating agent. By adjusting the pyrolysis temperature, the distribution of nitrogen functionalities and surface basicity were systematically tuned. The catalytic performance showed a strong dependence on the proportion of pyridinic-N species. Notably, the Ru/N-ABC-600 catalyst, containing the highest pyridinic-N content (37.7%), delivered 93.8% CO₂ conversion and 99.7% CH₄ selectivity at 380 °C. The superior activity was attributed to enhanced electron density and surface basicity introduced by nitrogen incorporation, which facilitated Ru0 formation for H₂ dissociation and strengthened CO₂ adsorption, thereby promoting CH₄ formation. Recently, Yang et al.^[147] reported the fabrication of Ni catalysts supported on biochar via a one-pot strategy employing metal-impregnated pine sawdust. First, the effect of pyrolysis temperature on CO₂ methanation activity was systematically examined at 400 °C. Notably, among the prepared samples, the catalyst carbonized at 600 °C (denoted as O-Ni-600) exhibited the best performance, achieving CO₂ conversions of 90%-98% and CH₄ yields of 90%-95%, as shown in Figure 7D and E. Moreover, the effect of the preparation method on CH₄ yield was investigated by comparing the performance of O-Ni-600, prepared via a one-pot method, with that of a catalyst prepared by post-impregnation (PI-Ni-600). As a result, O-Ni-600 showed superior activity and stability compared to PI-Ni-600. Under optimized reaction conditions, O-Ni-600 maintained outstanding catalytic performance over a 600 min methanation test, delivering CH₄ selectivity exceeding 91% [Figure 7F]. The superior activity of O-Ni-600 is attributed to the generation of finely dispersed Ni⁰ and NiO nanoparticles embedded within a porous carbon matrix during the one-pot process. The small particle size and intimate metal-support interaction promote efficient H₂ dissociation, while the high surface area and well-developed pore structure enhance CO₂ adsorption and activation.

Electrocatalytic CO₂ conversion

Electrochemical conversion of CO₂ has emerged as a promising strategy for decarbonizing chemical industries, particularly when driven by renewable electricity. The electrochemical CO₂ reduction reaction (CO₂RR) enables the generation of a wide range of value-added products, including CO, formate, CH₄, ethylene (C₂H₄), and ethanol (C₂H₅OH)^[148-151]. By tailoring synthesis and activation processes, biochar with tunable porosity, surface functionalities, and defect density can be engineered, making it a versatile platform for electrocatalytic CO₂ reduction.

One effective strategy involves integrating biochar into gas diffusion electrode (GDE) architectures to improve mass transport and interfacial charge transfer. Tan et al.^[152] reported biochar-modified GDEs based on coconut husk (CH)-derived biochar. In their configuration, a polytetrafluoroethylene membrane served as the gas diffusion layer, while Ag nanoparticles constituted the catalyst layer. An additional conductive biochar layer, obtained by pyrolyzing CH at 700-900 °C, was introduced to enhance electrical conductivity and facilitate interfacial charge transport [Figure 8A]. Among the prepared electrodes, Ag/CH-900 exhibited the highest conductivity and superior electrochemical performance. Although all electrodes reached current densities of 300 mA cm^-2, Ag/CH-900 required the least negative cathodic potential to achieve this value, indicating improved energy efficiency. Furthermore, it delivered CO faradaic efficiency (FE_CO) values of 94.4% at 50 mA cm^-2 and 86.5% at 100 mA·cm^-2, while maintaining stable operation for 8 h at 100 mA·cm^-2 [Figure 8B and C]. These results underscore the importance of conductive biochar layers in optimizing GDE configurations for high current density CO₂RR. Hu et al.^[153] successfully developed a molecular electrocatalyst by anchoring nickel phthalocyanine (NiPc) onto N-doped ball-milled biochar (NiPc/N-BMBC). The ball-milling process reduced particle size and exposed the internal carbon framework, generating a more accessible and defect-rich surface. Subsequent nitrogen doping further increased defect density and introduced additional coordination sites for immobilizing NiPc molecules. The strong electronic coupling between NiPc and N-doped biochar support enabled uniform molecular dispersion, facilitated charge transfer, and accelerated reaction kinetics, thereby improving CO₂ reduction activity. Specifically, the NiPc/N-BMBC achieved a FE_CO of 86% at -0.80 V vs. reversible hydrogen electrode (RHE), and a CO partial current density (j_CO) of 96 mA·cm^-2 at -1.0 V vs. RHE, outperforming both pristine NiPc and the N-BMBC. Furthermore, it maintained stable catalytic performance over 24 h of continuous electrolysis, demonstrating excellent durability and catalytic robustness. Recently, Zhang et al.^[154] synthesized N-doped biochar electrocatalysts derived from citrus peel, an abundant agricultural by-product rich in cellulose and oxygen-containing functional groups. The inherent porous structure and oxygen-rich composition of the precursor promoted in situ nitrogen incorporation and the formation of catalytically active sites during pyrolysis. FeCl₃ was introduced as a chemical activating agent to further enhance pore development and expose active sites, producing cost-effective N-doped biochar catalysts for CO₂ electroreduction. The optimized catalyst achieved an FE_CO of 89.8% at -0.62 V vs. RHE in an H-type cell [Figure 8D]. When implemented in a membrane electrode assembly (MEA) and operated at applied voltages between -2.5 and -3.25 V, it delivered a current density of -174 mA·cm^-2 with an FE_CO of 90.79% [Figure 8E and F]. These results highlight the strong potential of biomass-derived N-doped biochar for scalable and high-performance CO₂ electroreduction.

Figure 8. (A) Conceptual schematic of GDE using coconut husks-derived biochar. (B) FE_CO as a function of cathodic potential for Ag/CH-900. (C) Long-term stability test of Ag/CH-900. (A-C) Reprinted with permission^[152]. Copyright 2023, Elsevier. (D) FE_CO and FE_H2 for all prepared samples. (E) Schematic illustration of MEA cell. (F) FE_CO and corresponding current density of CP-NPC-3. (D-F) Reprinted with permission^[154]. Copyright 2025, Elsevier.

Additionally, porosity engineering has been demonstrated to play a crucial role in modulating catalytic performance. Zhang et al.^[155] reported the fabrication of biochar-derived Ni single-atom catalysts with precisely controlled pore architectures for efficient CO₂ electroreduction. By tailoring the activation process through different combinations of KHCO₃, KCl, and LiCl, microporous, mesoporous, and hierarchically porous carbon frameworks were constructed to anchor atomically dispersed Ni sites [Figure 9A]. In a typical synthesis, glossy privet biomass was first subjected to HTC, followed by high-temperature activation at 900 °C using distinct etching agents to generate porous biochar supports. Subsequent Ni ion adsorption and pyrolysis at 800 °C for 1 h yielded the corresponding Ni single-atom catalysts (hp-BioNC-Ni). Among the series, the hierarchically porous catalyst exhibited superior CO₂RR performance, achieving a maximum FE_CO of 97.4% at -1.2 V vs. RHE. Notably, it maintained FE_CO values above 85% over a broad potential window ranging from -0.7 to -1.7 V vs. RHE, significantly outperforming the counterparts [Figure 9B]. When compared with state-of-the-art carbon-based single-atom catalysts, this hierarchically porous system demonstrated one of the widest operational potential windows while retaining high CO selectivity [Figure 9C].

Figure 9. (A) Schematic representation of the preparation of biochar-derived single-atom Ni catalysts with tunable porosity. (B) FE_CO comparison of hp-BioNC-Ni, me-BioNC-Ni, mi-BioNC-Ni, and lp-BioNC-Ni. (C) Potential ranges of state-of-the-art carbon-based SACs. This figure quoted with permission^[155]. Copyright 2023, John Wiley and Sons.

Moreover, Wang et al.^[156] synthesized biochar-supported Au nanoparticle (Au NP) composites using swine manure as a biomass precursor. Due to its intrinsic abundance of heteroatoms such as nitrogen and oxygen, swine manure provides a favorable source for producing functional carbon materials. Manure-derived biochar was first obtained via hydrothermal treatment, followed by KOH activation to construct a porous carbon framework. Pre-synthesized Au nanoparticles were then uniformly deposited onto the activated biochar through ultrasonic-assisted dispersion, yielding a series of composites with different Au loadings, designated as Au/BC-0.1, Au/BC-0.6, and Au/BC-1.2. The electrocatalytic performance was evaluated in 1 M KOH electrolyte. Among the prepared catalysts, Au/BC-1.2 exhibited the highest CO selectivity, achieving a FE_CO of 87.72% at -0.4 V vs. RHE, along with a j_CO of 41.3 mA·cm^-2. Notably, Au/BC-0.1 delivered a remarkable mass activity of 9,451 A·g·Au^-1, highlighting the possibility of reducing noble metal loading while maintaining high catalytic performance.

Beyond selective CO production, biochar-based catalysts have been investigated for the generation of multicarbon (C₂₊) products such as C₂H₄ and C₂H₅OH^[157-161]. The tunable pore architecture and abundant surface functionalities of biochar can modulate intermediate adsorption and facilitate C-C coupling, a critical step in C₂₊ formation. For example, Yu et al.^[159] designed a Cu/CuO catalyst supported on phosphorus-enriched carbon obtained from Ceratophyllum demersum L. (denoted as Cu-CuO/P-CCDL) for the selective electrochemical reduction of CO₂ to C₂H₅OH. The biomass precursor inherently contains substantial amounts of carbon, oxygen, and phosphorus within its biological framework, enabling direct phosphorus incorporation into the carbon matrix during carbonization without the need for external dopants. Electrocatalytic measurements were conducted in an H-type cell using CO₂-saturated 0.5 M NH₄Cl as the electrolyte. Under these conditions, the Cu-CuO/P-CCDL-600 catalyst achieved a C₂H₅OH selectivity of 38.9% at -1.5 V vs. RHE. Additionally, it exhibited a favorable faradaic efficiency distribution, with a higher proportion of C₂₊ products relative to competing C₁ species and hydrogen evolution. The enhanced CC coupling capability was attributed to the synergistic interaction between Cu/CuO active sites and the phosphorus-modified carbon framework.

Chen et al.^[160] employed locust leaves as a biomass precursor to fabricate high-performance Cu-based electrocatalysts for CO₂ reduction. The biomass-derived carbon was first activated with K₂CO₃ and pyrolyzed under an N₂ atmosphere to produce porous biochar. Subsequently, a series of Cu_xO@C composites were prepared via a solvothermal method, in which the carbon content was systematically adjusted by varying the amount of introduced biochar. The electrocatalytic performance was evaluated in an H-type cell using 0.5 M KHCO₃ as the electrolyte. Among the series, Cu_xO@C-2, exhibited the highest FE_C2+ of 41.3% at -1.08 V vs. RHE, approximately 1.7 times higher than that of the pristine Cu_xO catalyst. To further elucidate the interaction between the catalyst surface and reaction intermediates, single oxidative scans were conducted to probe OH^- adsorption behavior. Cu_xO@C-2 displayed the most negative OH^- adsorption potential (-0.71 V vs. RHE), followed by Cu_xO@C-3 (-0.66 V), Cu_xO@C-1 (-0.60 V), and pristine Cu_xO (-0.55 V). The more negative adsorption potential indicates stronger binding affinity toward OH^- species, suggesting more favorable intermediate stabilization in Cu_xO@C-2. Hu et al.^[161] developed Cu-based electrocatalysts for selective C₂₊ production using pig manure-derived biochar as a carbon support prepared via a hydrothermal strategy. The optimized catalyst, denoted as Cu-NB-2000, achieved an FE_C2+ of 47.35% with a j_C2+ of 241.2 mA·cm^-2 at -1.51 V vs. RHE. In addition to its high activity, the catalyst demonstrated good durability, maintaining stable current density and FE_C2+ during 8 h of continuous electrolysis at -0.91 V vs. RHE.

Photocatalytic CO₂ conversion

Solar-driven CO₂ reduction into value-added fuels has attracted significant attention as a sustainable strategy to address energy shortages and climate change. Nevertheless, the practical efficiency and product selectivity of photocatalytic CO₂ conversion are often limited by the high thermodynamic barrier for C=O bond activation and the rapid recombination of photogenerated charge carriers. Therefore, the rational design of photocatalysts with optimized electronic structures and efficient charge-separation pathways is essential to improve CO₂ reduction performance. In this context, biochar has emerged as a promising component in photocatalytic systems. Owing to its intrinsic electrical conductivity and tunable surface chemistry, biochar can serve as an electron mediator or conductive scaffold in semiconductor-based composites, effectively suppressing charge recombination. This conductive network facilitates rapid electron transport, reduces interfacial charge-transfer resistance, and enhances overall photocatalytic activity.

For example, Lv et al.^[162] reported the hydrothermal synthesis of a CuS-loaded NBC composite derived from corncob powder. Electrostatic attraction between negatively charged NBC and positively charged CuS in aqueous solution enabled uniform immobilization of CuS nanoparticles on the biochar surface, forming well-defined heterointerfaces. Zeta potential measurements [Figure 10A] confirmed the opposite surface charges of the two components, supporting their spontaneous assembly. The resulting interfacial junctions generated localized electric fields that promoted directional charge transfer and improved the utilization of active sites. Consequently, the CuS/NBC composite achieved a CO production rate of 2.85 μmol·g^-1·h^-1 under 4 h of continuous illumination, significantly surpassing that of pristine CuS (0.82 μmol g^-1·h^-1) and NBC (0.83 μmol· g^-1·h^-1) [Figure 10B]. Moreover, the optimized CuS/NBC sample exhibits significant performance stability during four cycles under light illumination, with a total test duration of 16 h [Figure 10C]. The enhanced performance was attributed to more efficient separation and migration of photogenerated charge carriers at the CuS/NBC interface, leading to improved CO₂ activation and reduction. Lin et al.^[163] developed amphiphilic CoO_x/biochar (CDB) photocatalysts using loofah sponge as a biomass-derived template for CO₂ photoreduction. The loofah precursor, characterized by its highly porous structure and strong ion-adsorption capability, was first impregnated with cobalt ions and subsequently calcined under an N₂ atmosphere to obtain CDB materials with different CoO_x loadings. Among the synthesized samples, the composite containing 3.0 wt% CoO_x (CDB-3) exhibited irregular micro-/nanostructures that imparted pronounced amphiphilic properties. Such surface characteristics enhanced CO₂ diffusion within the reaction medium and facilitated mass transport toward active sites. As a result, CDB-3 achieved a CO production rate of 48.3 μmol·h^-1 with a turnover frequency of 0.29 s^-1, highlighting the importance of amphiphilic surface design in improving photocatalytic CO₂ conversion. Furthermore, Yu et al.^[164] constructed a facile biochar-assisted S-scheme CeO₂-based heterojunction photocatalyst, denoted as CN-BIO/CeO₂. In this system, strong interfacial coupling between CeO₂ and g-C₃N₄ generated an internal electric field characteristic of the S-scheme architecture, while conductive biochar served as an efficient electron-transport pathway. As a result, photogenerated electrons were rapidly extracted through the biochar framework rather than accumulating on the semiconductor surface, effectively suppressing charge recombination and enhancing redox capability during CO₂ photoreduction. The photocatalytic performance was evaluated under full-spectrum simulated solar irradiation in a gas-solid reaction system. Pristine g-C₃N₄ and CeO₂ exhibited low CO production rates of 5.98 and 5.21 μmol·g^-1, respectively. In comparison, the CN/CeO₂ produced 22.2 μmol·g^-1 of CO, whereas CN-BIO/CeO₂ achieved a significantly higher yield of 69.1 μmol·g^-1 with a CO selectivity of approximately 95%, the highest among the tested samples. Moreover, the catalyst maintained stable activity over four consecutive cycles, indicating good durability.

Figure 10. (A) Comparison of zeta potential between NBC and CuS. (B) Photocatalytic CO and H₂ production yields of the as-synthesized samples. (C) Catalyst recyclability of CuS/NBC compared with CuS. This figure quoted with permission^[162]. Copyright 2026, Elsevier.

Yang et al.^[165] prepared a cherry blossom-derived biochar-modified Co-Al layered double hydroxide photocatalyst (Co-Al LDH-C) via an ultrasonic-assisted solvent treatment. Compared with pristine Co-Al LDH, the biochar-integrated composite demonstrated markedly enhanced CO₂ reduction performance. Under optimized conditions, Co-Al LDH-C achieved a CO production rate of 29.2 µmol·g^-1 approximately 15 times higher than that of the unmodified Co-Al LDH (2.1 µmol·g^-1). The enhanced activity was attributed to improved light absorption, accelerated charge separation, and reduced carrier transport resistance induced by biochar incorporation, which collectively increased the density of accessible active sites. Furthermore, only a slight decrease in CO evolution was observed after 30 h of continuous operation, confirming satisfactory long-term stability. Structural and spectroscopic analyses indicated that the synergistic interaction between biochar and Co-Al LDH was critical for promoting photocatalytic CO₂ conversion. Zhang et al.^[166] synthesized NiO nanoparticle-decorated activated pinecone-derived biochar (NiO/APC) composites via a two-step strategy. Pinecones were first carbonized and chemically activated with KOH to construct a tubular porous carbon framework, where gaseous species generated during activation created abundant pore structures. The surface functional groups formed during this process promoted the adsorption of Ni²⁺ ions, which were subsequently converted into NiO nanoparticles through heat treatment at 300 °C under oxygen-deficient conditions. Photoluminescence (PL) spectra revealed that the NiO/APC composites exhibited markedly lower emission intensities than pristine NiO, with NiO/APC-3 showing the weakest signal. The suppressed PL intensity indicates reduced charge recombination and improved separation of photogenerated carriers, attributed to efficient electron transfer across the conductive APC support. Consequently, NiO/APC-3 demonstrated markedly enhanced photocatalytic activity, achieving CH₄ and CO production yields of 81.06 and 15.00 μmol·g^-1, respectively, which are 66.4 and 1.9 times higher than those of pristine NiO, and 2.0 and 1.2 times higher than those of NiO/PC.

Table 3 highlights that product selectivity differs markedly among thermocatalytic, electrocatalytic, and photocatalytic CO₂ conversion pathways. While pore architecture influences local CO₂ enrichment, reactant transport, and active-site accessibility, product selectivity is ultimately governed by the interplay between the supported active centers and their interactions with key reaction intermediates. Therefore, rational engineering of biochar porosity and surface chemistry should be tailored to the specific reaction pathway rather than relying on a single structural parameter.