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].

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 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.

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.

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 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.

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.

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.

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.

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.

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.

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].

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.

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.

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.