Biomass-Derived Nanoengineered Carbon Materials for Environmental Remediation and CO₂ Valorization

Abstract

Biomass-derived nanoengineered carbon materials have emerged as key platforms in environmental technologies due to their high surface area, electrical conductivity, chemical stability, and sustainable synthetic route starting from renewable feedstock. This broad family comprises dimensionally nanoscale materials, such as carbon dots, carbon nanofibers, and graphene-based structures, as well as biochars, hydrochars, activated carbons, and related porous carbonaceous materials whose pore architecture, surface chemistry, or defects are deliberately engineered at the nanometer scale. Beyond their traditional role as passive supports, these materials can actively regulate adsorption phenomena, charge transport, and catalytic microenvironments through precise control of heteroatom doping, graphitic domains, and hierarchical porosity. Among current environmental priorities, carbon dioxide (CO₂) management represents one of the most pressing challenges. Biomass-derived nanocarbons offer tunable adsorption sites for selective CO₂ capture while simultaneously serving as active matrices for catalytic conversion. Tailored doped-carbon frameworks can stabilize key reaction intermediates, suppress competing pathways such as hydrogen evolution, and promote selective transformation into fuels and high-value chemicals. In addition, these materials are excellent hosts for atomically dispersed metals, dual-site catalysts, and semiconductor hybrids used in electrochemical and photocatalytic CO₂ reduction. By combining renewable sourcing with nanoscale control of reactivity, carbon materials create a bridge between environmental remediation and carbon valorization. This review critically examines recent progress in biomass-derived nanoengineered carbon materials for integrated CO₂ capture and conversion, with emphasis on structure-property-performance relationships, mechanistic roles, scalability, and sustainability. Particular attention is also devoted to catalytic conversion and electrochemical CO₂ sensing, where carbon-based and hybrid interfaces enable the transduction of CO₂ recognition into measurable electrical responses. These materials represent a promising yet underexplored pathway toward circular carbon management and the development of next-generation low-carbon chemical technologies.

1. Introduction

Carbon nanomaterials (CNMs) and porous carbonaceous materials have emerged as highly versatile platforms for environmental technologies owing to their high specific surface area, porosity, chemical stability, and excellent electrical conductivity. These properties enable efficient adsorption, charge transport, and surface functionalization through heteroatom doping and defect engineering, making these nanomaterials attractive for pollutant removal, catalysis, sensing, and electrochemical applications [1,2,3].

Carbon nanomaterials comprise structurally distinct classes whose dimensionality and organization determine their functional roles. Zero-dimensional (0D) materials, including carbon quantum dots and graphene quantum dots, are characterized by nanoscale dimensions, abundant edge sites, tunable photoluminescence, and efficient charge-transfer properties [4,5]. These characteristics make them particularly suitable for optical and electrochemical sensing, photocatalysis, supercapacitors, rather than for bulk carbon dioxide (CO₂) adsorption. One-dimensional (1D) nanostructures, such as nanotubes and nanowires, exhibit nanoscale confinement across two spatial directions while extending along the third, thereby providing elongated geometries, large accessible surfaces, and preferential pathways for charge transport [6]. Two-dimensional (2D) graphene, graphene oxide, and reduced graphene oxide offer accessible surfaces, defect-rich basal planes, and strong in-plane conductivity, which are advantageous for sensing, optoelectronic or electrocatalytic applications, although restacking can reduce their accessible surface area [6].

Biochar, hydrochar, activated carbon, and related porous carbon frameworks constitute a distinct class of carbonaceous materials and should not automatically be classified as CNMs. Nevertheless, these materials can be nanoengineered through controlled thermochemical routes to regulate their internal architecture, surface chemistry, defects, and catalytic interfaces at the nanometer scale. Such porous carbonaceous materials are generally more suitable for carbon dioxide (CO₂) capture because surface functionalities provide high adsorption capacity [7,8]. Their chemically modifiable surfaces also allow them to serve as catalytic supports and electrochemical interfaces.

However, not every carbon material is equally suitable for every application: CO₂ adsorption at low pressure is primarily governed by ultramicropore volume, pore-size distribution, and surface chemistry [9,10]. By contrast, electrochemical conversion requires electronically conductive frameworks, accessible catalytic sites, and effective gas–liquid–solid interfaces [11]. Sensing applications additionally require the coupling of a selective recognition component with an efficient electrical or optical transduction pathway [12].

Growing interest in sustainable materials has encouraged the production of carbon nanomaterials from renewable biomass and waste resources within circular economy and green chemistry frameworks. Compared with fossil-derived carbons, biomass-derived CNMs combine low-cost feedstocks with reduced environmental impact while offering remarkable flexibility for tailoring pore architecture, surface functionality, and electronic properties [13]. Such versatility has expanded their application beyond conventional adsorption toward catalysis, sensing, and electrochemical technologies. Nevertheless, rigorous assessment of their environmental benefit requires life-cycle perspectives that consider energy demand, reagent consumption, activation steps, and overall process impacts. Among current environmental priorities, CO₂ management has become one of the most urgent scientific and technological challenges. Global energy-related CO₂ emissions reached a record 37.8 Gt in 2024, while atmospheric concentrations approached 422.5 ppm, highlighting the urgent need for effective mitigation strategies [14]. Consequently, research has progressively shifted from conventional CO₂ capture toward integrated approaches capable of combining selective capture with catalytic conversion into fuels and value-added chemicals using renewable or low-carbon energy inputs [15].

CO₂ valorization extends beyond conventional carbon capture by treating carbon dioxide as a renewable C1 feedstock for the production of fuels, chemicals, and carbon-containing materials. Current conversion strategies include thermocatalytic hydrogenation, electrochemical reduction, photocatalytic conversion, and integrated capture–conversion processes. Recent advances in catalyst design, reactor engineering, and renewable-energy integration have improved reaction rates and product selectivity, enabling the conversion of CO₂ into products such as carbon monoxide, formic acid, methanol, methane, and other carbon compounds. Nevertheless, the industrial implementation of these technologies remains limited by the high thermodynamic stability of CO₂, substantial energy requirements, low single-pass conversion, competing reaction pathways, catalyst deactivation, and the need for efficient product separation. Consequently, the development of multifunctional materials capable of combining CO₂ enrichment, activation, and selective conversion is increasingly regarded as a key strategy for improving the technical and economic feasibility of CO₂ valorization [16].

In this context, carbon nanomaterials can play a dual strategic role. First, they function as selective sorbents for CO₂ capture, where microporosity, hierarchical pore networks, and surface functionalities regulate adsorption capacity, selectivity, and regeneration behavior. Second, they act as active components or conductive scaffolds in electrochemical and photocatalytic CO₂ conversion systems, where conductivity, defect density, graphitic domains, and heteroatom sites promote charge transfer and stabilize key reaction intermediates. Representative evidence was reported by Hao et al., who transformed low-cost wood biomass into a three-dimensional N-doped graphitized carbon material (5.38% N; 1673.6 m² g⁻¹), achieving 91% Faradaic efficiency for electrochemical CO₂ reduction with operational stability of at least 20 h [17]. Similarly, recent photocatalytic studies have demonstrated that biomass-derived carbon dots can function as efficient sensitizers and charge mediators in hybrid semiconductor systems, improving light harvesting and charge separation during CO₂ photoreduction [18].

Although biomass-derived carbon nanomaterials have emerged as versatile platforms for integrated CO₂ capture and conversion, other classes of nanomaterials have also been extensively investigated for the same applications. In particular, zeolites, metal–organic frameworks (MOFs), metal oxides, and MXene-materials have demonstrated excellent performance depending on the target process [19,20,21]. Nevertheless, biomass-derived carbon materials have attracted increasing attention because they combine competitive functional performance with renewable feedstocks, scalable synthesis, and lower environmental impact, making them particularly attractive for sustainable carbon management. Despite these advances, major challenges remain. These include bridging the gap between laboratory-scale performance and real operating environments, engineering structures that simultaneously optimize capture and catalytic conversion, ensuring long-term material stability, and scaling green synthesis methods in an economically viable manner.

Unlike previous reviews that primarily focus on individual classes of carbon materials or specific environmental applications, the present work provides an integrated structure–property–performance perspective spanning biomass-derived nanoengineered carbon materials for CO₂ capture, catalytic valorization, and electrochemical sensing. Particular emphasis is placed on correlating synthesis strategies with physicochemical properties, functional performance, sustainability metrics, and industrial scalability, providing a comprehensive framework for the rational design of next-generation carbon materials for sustainable carbon management.

2. Scope and Synthesis Framework

This section defines the scope of the review and outlines the main strategies for converting biomass into nanoengineered carbon materials. In addition to important sustainability considerations, special emphasis is given to how precursor selection, synthesis pathways, and post-treatment strategies interact to determine the structural and functional qualities of the final materials.

2.1. Literature Search Strategy

This narrative review primarily considers peer-reviewed studies published between 2021 and 2026, identified in Scopus, Web of Science, and Google Scholar using combinations of keywords including “CO₂,” “biomass-derived carbon materials,” “carbon nanomaterials,” “biochar”, “hydrochar”, and “porous carbon.” Studies were selected based on relevance and availability of structural or quantitative performance data, while earlier publications were included when providing foundational concepts or representative benchmarks.

2.2. From Biomass to Nanoengineered Carbon Materials: Sustainable Precursors and Conversion Routes

Conventional fossil-derived carbon nanomaterials generally require energy-intensive synthesis routes, such as chemical vapor deposition, laser ablation, chemical oxidation, or exfoliation, often involving strong acids or other hazardous chemical reagents [22]. These methods typically produce highly crystalline materials with excellent electrical conductivity and structural uniformity, making them particularly suitable for electronics and high-performance electrochemical devices. However, they usually require high temperatures, purified precursors, and energy-intensive processing, resulting in higher production costs and a greater environmental footprint [23,24].

Although biomass-derived carbons may exhibit greater structural heterogeneity and lower degrees of graphitization than their synthetic counterparts, their properties can be readily tailored for specific applications, particularly adsorption processes and environmental remediation. In particular, biomass-derived carbon nanomaterials exploit intrinsic lignocellulosic structures and the naturally occurring heteroatom content (e.g., N, S, and P), often reducing the need for hazardous chemical treatments while facilitating the development of tailored surface functionalities without extensive post-functionalization. At the same time, they promote waste valorization and support circular carbon management strategies. Therefore, biomass-derived carbon nanomaterials typically exhibit abundant active sites, ease of surface modification and tunable chemical propertiesmaking them candidates for adsorption, heterogeneous catalysis, and environmental pollutants detection, whereas conventionally synthesized carbon nanomaterials are often preferred for applications requiring high crystallinity and electrical conductivity. The physicochemical properties biomass-derived nanoengineered carbon materials are primarily governed by the interplay between precursor composition and conversion strategy. Lignocellulosic biomass provides an intrinsic source of oxygen-containing groups, mineral species and naturally occurring heteroatoms that strongly influence carbon yield, pore development, surface chemistry and electronic properties during carbonization [25,26,27]. Consequently, biomass conversion should not be regarded as a simple carbonization process but as a versatile platform for engineering carbon nanoarchitectures tailored for CO₂ capture, catalytic conversion and sensing. For example, lignin-based and carbohydrate-derived routes combined with post-treatments (activation and N-doping) have delivered N-functional porous carbons specifically targeted for CO₂ capture, with performance governed by the synthesis sequence [28], as shown in Figure 1.

The conversion of biomass into CNMs generally involves thermochemical or physicochemical routes such as hydrothermal carbonization, pyrolysis, microwave-assisted conversion, combustion-assisted processes, and subsequent physical or chemical activation. Each pathway provides different degrees of control over pore architecture, graphitic ordering, oxygen-containing functionalities, heteroatom incorporation, and electrical conductivity [17]. Therefore, the synthesis route should not be considered only as a preparative step, but as a key design tool for tuning the structure and functionality of the resulting carbon material [29].

Accordingly, in this section, the main biomass-to-carbon conversion strategies are discussed with emphasis on how processing conditions affect the structural and chemical features of the resulting CNMs. Particular attention is given to hydrothermal carbonization, microwave-assisted conversion, controlled pyrolysis/combustion, and activation strategies, as these routes provide complementary opportunities for tailoring biomass-derived carbon materials for environmental and CO₂-related applications. Moreover,

Table 1. Representative biomass-derived carbon nanomaterials and carbonaceous materials for CO₂ capture and conversion.

Biomass/Waste Precursor	Synthesis Route	Activation/Functionalization	Main Structural Features	CO2-Related Application	Representative Performance	Key Limitation/Critical Note	Refs.
Microcrystalline cellulose	Hydrothermal carbonization at 200 °C followed by chemical activation	KOH activation	Activated hydrochar microspheres; developed microporosity	CO2 adsorption	CO2 uptake up to 7.07 mmol g−1 at 25 °C	Chemical activation may increase reagent consumption and washing demand	[30]
D-glucose	Hydrothermal treatment followed by N-doping and activation	Urea treatment and K2CO3 activation	N-doped porous carbon; tunable basic sites	CO2 adsorption from combustion exhaust streams	Enhanced CO2 adsorption through combined porosity and N-functionality	Model precursor, less representative of real biomass variability	[31]
Coffee-derived biowaste	HTC-derived hydrochar followed by activation	Solvent washing/purification before activation	Pore-engineered activated hydrochar; modified ultramicroporosity	Post-combustion CO2 capture	Solvent-driven pore engineering significantly affects capture performance	Downstream purification can alter environmental footprint	[32]
Cedar wood biomass	HTC-assisted synthesis followed by high-temperature treatment	Melamine and FeCl3-assisted N-doping/graphitization	3D N-doped graphitized carbon; 5.38% N; 1673.6 m2 g−1	Electrochemical CO2 reduction to CO	91% Faradaic efficiency toward CO; stability ≥ 20 h	Stability still evaluated over relatively short laboratory times	[17]
Corn straw	Microwave-assisted pyrolysis	H3PO4 pre-activation followed by KOH etching	Hierarchical porous biochar; tuned pore network	CO2 adsorption	Optimized CO2 uptake through balance between transport and adsorption kinetics	Multi-step activation increases process complexity	[33]
A. marmelos leaf extract	Integrated microwave–HTC route	Coupling with phytochemical CdS quantum dots	Biomass-derived carbon dots embedded in semiconductor hybrid	Photocatalytic CO2 reduction to HCOOH	Improved charge separation and interfacial reaction control	Hybrid photocatalyst stability and recovery require evaluation	[18]
Soybean cake	Pyrolysis followed by chemical activation	KOH activation	Industrial biomass-waste-derived porous carbon	CO2 adsorption; CO2/N2 and CO2/CH4 separation	Evaluated for CO2 uptake and gas selectivity	Activation severity and yield should be reported systematically	[34]
Sawdust/wood waste	Pyrolysis/activation	Physical or chemical activation	Porous activated carbon; surface functional groups	CO2 capture	Scalable waste-derived adsorbent platform	Performance depends strongly on precursor composition and activation protocol	[35]
Biomass-derived carbon	Molten-salt synergistic activation	KHCO3–KCl activation	High surface area; abundant ultramicropores < 0.7 nm; oxygen-containing groups	CO2 capture	Improved adsorption capacity, CO2/N2 selectivity, kinetics and cyclic stability	Salt recovery and washing steps should be considered	[36]
Cotton fibers	One-step chemical activation	Alkali activation	Ultramicroporous activated carbon; sub-nanometer pore regulation	CO2 adsorption	Efficient CO2 adsorption through optimized ultramicropore window	Optimization sensitive to alkali/carbon ratio and activation temperature	[37]
Biomass-derived activated carbons	KOH, K2C2O4 or mixed activation	Controlled activation chemistry	High surface area and microporosity; tunable surface functionality	CO2 capture and CO2/N2 selectivity	Up to ~5.95 mmol g−1 at 25 °C and 9.65 mmol g−1 at 0 °C	Trade-off between high uptake, burn-off and chemical consumption	[38]
Olive-kernel biochar	Biochar preparation followed by metal catalyst deposition	Ni-based catalytic phase on biochar support	Carbon support promoting metal dispersion and stability	Thermocatalytic CO2 methanation	72% CO2 conversion and 95.5% CH4 selectivity	Requires high-temperature operation and long-term deactivation studies	[39]
Engineered biochar	Biochar-supported metal catalyst preparation	Co nanoparticles; Ce promotion	Metal nanoparticles dispersed on biochar	CO2 methanation	Up to 60% CO2 conversion; CH4 selectivity approaching 80% with Ce	Metal loading, sintering and support stability require optimization	[40]
Carbon nanofiber-based electrode	Electrospinning/carbonization route	SnO2 embedded in carbon nanofibers	Self-supporting conductive carbon framework	Electrochemical CO2 reduction to formate	Faradaic efficiency > 90% toward formate	Long-term stability and scale-up of electrode architecture remain critical	[41]
Carbon nanodots from waste biomass	Biomass conversion to carbon nanodots	Metal-free carbon nanodot preparation	Nanoscale carbon dots; photoactive/electron-mediating features	Photocatalytic CO2 reduction to methanol	Selective CO2 photoreduction into methanol	Product selectivity, quantum efficiency and scale-up require further validation	[13]

provides an initial overview of representative systems, illustrating how biomass precursor selection, conversion route, and post-treatment collectively determine material structure, functional properties, and performance in CO₂ capture and conversion.

2.2.1. Hydrothermal Carbonization (HTC)

Hydrothermal carbonization (HTC) is one of the most versatile routes for converting wet biomass into carbonaceous materials because it operates under relatively mild temperatures (160–300 °C) while avoiding energy-intensive drying steps. HTC primarily produces hydrochar rather than a dimensionally nanoscale carbon material; subsequent activation, doping, or size-controlled processing is therefore required to obtain nanoengineered porous structures or CNMs. Indeed, the final properties of HTC-derived carbons strongly depend on reaction temperature, residence time and post-treatment strategy. Chemical activation, nitrogen incorporation and purification treatments progressively transform hydrochars into highly porous materials with tailored adsorption sites and improved electronic properties.

Several studies illustrate this design flexibility. For instance, hydrochars obtained from microcrystalline cellulose and subsequently activated with KOH reached CO₂ adsorption capacities up to 7.07 mmol g⁻¹ at 25 °C [30], whereas glucose-derived hydrochars combined with urea treatment and K₂CO₃ activation generated nitrogen-enriched porous carbons with enhanced affinity toward combustion-derived CO₂ in post-combustion flue gases [31]. More recently, coffee-waste hydrochars demonstrated that solvent purification can also act as an effective pore-engineering step [32], while cedar-derived hydrochars further extended HTC applications to electrochemical CO₂ reduction to CO at relatively low overpotential and maintaining stability for at least 20 h, achieving Faradaic efficiencies of 91% through nitrogen-assisted graphitization [17] (with melamine and FeCl₃ assistance).

2.2.2. Microwave-Assisted Conversion

Compared with conventional heating, microwave-assisted conversion provides rapid volumetric heating, shorter reaction times and improved energy efficiency. These features facilitate homogeneous carbonization while promoting pore development and defect formation. Depending on the activation strategy, microwave processing can generate either highly porous adsorbents or electronically active carbon nanostructures. A clear example is the work of Qiu et al. [33], where corn straw was converted into porous biochar via microwave-assisted pyrolysis, followed by a two-step activation (pre-activation with H₃PO₄ and chemical etching with KOH), optimizing CO₂ adsorption capacity by balancing transport and kinetics. Similarly, biomass-derived carbon dots obtained from A. marmelos leaf extract through an integrated microwave–hydrothermal process were coupled with phytochemically synthesized CdS quantum dots and applied to the photocatalytic reduction of CO₂ to formic acid (HCOOH). This example illustrates the role of dimensionally nanoscale carbons in promoting charge separation and controlling interfacial reactions within hybrid photocatalysts [18].

2.2.3. Controlled Pyrolysis and Combustion

Controlled pyrolysis remains one of the most widely employed thermochemical routes for converting biomass into carbon nanomaterials. Performed under an inert atmosphere, the process enables the progressive devolatilization of lignocellulosic precursors while preserving the carbon framework. The physicochemical properties of biomass-derived nanoengineered carbon materials are primarily governed from pore architecture, graphitic ordering, surface area, surface functionality and electrical conductivity, are strongly influenced by pyrolysis temperature, heating rate, residence time and post-treatment strategy [33]. Consequently, careful control of these parameters enables the rational design of carbon materials for CO₂ capture and catalytic applications.

Although pyrolysis produces the carbon framework, subsequent activation has a major role in the ultimate adsorption and catalytic activity [32]. Chemical or physical activation tailors micropore development, surface chemistry and mass transport, thereby optimizing biomass-derived carbons for different CO₂-related applications. The versatility of pyrolysis-derived carbons is illustrated by a wide range of biomass feedstocks: for instance, soybean cake has been converted into porous carbons exhibiting high CO₂ adsorption capacity together with enhanced CO₂/N₂ and CO₂/CH₄ selectivity, highlighting the relevance of activation chemistry for gas separation [34]. Likewise, sawdust-derived activated carbons represent scalable adsorbents whose performance depends on the combined effects of precursor composition and activation protocol [35]. More recently, combustion-assisted restructuring of recycled fiberboards has enabled the formation of graphene-like porous nanosheets, illustrating how highly exothermic treatments can also be exploited to engineer carbon nanoarchitectures [42].

2.2.4. Sustainability Metrics and Techno-Economic Considerations

Although biomass-derived carbon materials are generally regarded as sustainable because they originate from renewable feedstocks, their overall environmental performance depends on the entire production chain rather than on the precursor alone. In many cases, the environmental footprint is governed by energy consumption, activation chemistry, reagent recovery, washing steps, wastewater generation and regeneration requirements rather than by the biomass source itself. Consequently, evaluating the sustainability of carbon nanomaterial production requires a holistic approach that integrates material performance with process efficiency. Therefore, adsorption capacity should always be interpreted together with process-related indicators such as carbon yield, burn-off, activator-to-precursor ratio, reagent consumption, regeneration energy and working capacity under cyclic operation. For instance, KOH activation is highly effective for generating ultramicroporosity and maximizing CO₂ uptake, but it is also associated with significant burn-off, high chemical consumption and extensive washing requirements. Similarly, Kusdhany et al. [32] demonstrated that solvent washing of hydrochars prior to activation significantly modifies pore-size distribution and, consequently, CO₂ adsorption performance, highlighting that purification is not merely a cleaning step but an effective pore-engineering strategy. These findings emphasize that downstream processing can substantially influence both material performance and overall process sustainability. Therefore, comparisons among biomass-derived carbons should include both adsorption performance and process efficiency metrics to avoid overestimating the sustainability of chemically intensive routes [32,33].

To address these challenges, Life-Cycle Assessment (LCA) and Techno-Economic Assessment (TEA) are increasingly employed as complementary tools for evaluating the industrial feasibility of biomass-derived carbon materials. Rather than serving as post-development analyses, these approaches should be integrated into material design to identify environmental hotspots, optimize reagent consumption, and assess economic viability. A representative example is the integrated LCA/TEA study by Lee et al. [43], who evaluated activated carbons produced from five waste feedstocks for post-combustion CO₂ capture. Although chemically activated carbons exhibited superior adsorption performance, their production costs were largely driven by KOH consumption. The minimum selling price was 7.97, 6.70, 6.28, 3.76, 4.14, and 3.63 USD kg⁻¹ for sawdust, jujun grass, Arundo donax, coconut husk, and palm kernel shell, respectively. Nevertheless, all investigated materials offset their production-related emissions during operation, while replacing grid electricity with renewable energy reduced life-cycle emissions by up to 72%.

Ultimately, the development of sustainable biomass-derived porous and carbon nanomaterials requires balancing functional performance with material yield, resource efficiency, environmental impact, and economic viability rather than maximizing a single metric. Representative techno-economic, life-cycle and life-cycle-costing studies are compared in the following

Table 2. Representative techno-economic and life-cycle assessments relevant to carbon materials for CO₂ capture and conversion.

System	Assessment Scope	Application Basis	Key Economic Indicator	Main Cost and Environmental Drivers	Main Implication	Ref.
Waste-derived activated carbons from six feedstocks for post-combustion CO2 capture.	Integrated LCA and TEA, including activated-carbon production, use-phase carbon offset and electricity scenarios.	Production and application of waste-derived activated carbons for post-combustion CO2 capture.	Minimum selling price: 3.63–7.97 USD kg−1, depending on feedstock and process yield.	KOH consumption was the dominant production-cost driver; the electricity source strongly affected life-cycle emissions.	All assessed carbons offset their production-related emissions during operation. Renewable electricity reduced life-cycle emissions by up to 72%.	[43]
Activated carbon and Cu-modified activated carbon.	LCA coupled with life-cycle costing, cumulative energy demand, ecological footprint and CO2 adsorption.	Production and use of activated carbon and modified activated carbon for the adsorption of 1 kg CO2.	Production cost: 1.6 USD kg−1 for activated carbon and 2.0 USD kg−1 for Cu-modified activated carbon.	Equipment and construction accounted for >23% of the economic burden. H3PO4 and Cu inputs contributed 97% and 61.48% of the respective environmental burdens.	Cu-modified activated carbon caused significant life-cycle impacts and needed a 2.6-fold higher cumulative energy demand, despite the fact that treatment enhanced CO2 adsorption.	[44]
Coal- and biomass-derived activated-carbon.	Life-cycle sustainability assessment integrating environmental, economic and energy-efficiency dimensions.	Five activated-carbon production technologies: two coal-derived and three biomass-derived routes.	Comparative production-cost assessment of the five production routes.	Activation method, energy supply, raw-material consumption and process yield governed the overall sustainability performance.	Steam-activated kernel-shell carbon showed the most favorable environmental profile, demonstrating that biomass origin alone does not guarantee superior sustainability.	[15]
Direct electrochemical conversion of captured CO2 in an amine-based solvent to CO.	Process-level TEA and LCA comparing captured-CO2 reduction with conventional CO2 reduction.	Cost and life-cycle impacts of the produced CO.	The levelized CO cost of captured CO2 reduction was 3.5 times higher at current performance; at technological parity, it was predicted to be 6.1% lower than conventional CO2 reduction.	Electricity supply, capture-conversion integration, and electrolyzer performance.	By avoiding CO2 emission and solvent-regeneration penalties, direct capture-conversion integration could become competitive; renewable electricity is necessary for environmental benefits.	[45]

. Because the selected studies employ different functional units and system boundaries, their numerical results should be interpreted as study-specific indicators rather than as directly interchangeable benchmarks.

3. Structure-Function Design Principles Enabling CO₂ Capture, Adsorption and Catalytic Conversion

The performance of biomass-derived carbon nanomaterials in CO₂ capture, catalytic conversion and electrochemical sensing is governed by the synergistic interplay between their structural, chemical and electronic properties [46,47,48]. Among porous materials, biomass-derived carbon nanomaterials offer remarkable versatility because their pore architecture, surface chemistry and electronic structure can be tuned simultaneously through precursor selection and synthesis strategy [49,50]. Rather than acting as inert supports, these materials create engineered interfacial microenvironments that regulate CO₂ adsorption, mass transport, charge transfer and catalytic activity [51,52]. Consequently, their performance cannot be attributed to a single descriptor, such as surface area, but to the cooperative contribution of multiple structural features. Figure 2 summarizes the fundamental structure–function relationships discussed throughout this section, illustrating how structural, chemical and electronic features collectively determine the performance of biomass-derived carbon nanomaterials in CO₂-related applications.

This section examines the fundamental structure–function relationships governing biomass-derived carbon nanomaterials, providing the conceptual framework for the CO₂ capture, catalytic conversion and electrochemical sensing applications discussed in the following sections.

In these materials, performance is not governed by a single descriptor, such as surface area, but by the cooperative interaction among ultramicroporosity, hierarchical pore architecture, heteroatom doping, defect density, graphitic ordering and surface chemistry. Ultramicropores enhance CO₂ confinement and low-pressure adsorption, while heteroatom functionalities and defect sites modulate local charge distribution, surface polarity and adsorption energetics [10,17,54,55,56,57,58,66] At the same time, graphitic domains and electronically active dopants promote charge transport and interfacial electron transfer, which are essential for electrochemical and photocatalytic CO₂ conversion [59,60,61,62,63,64]. Therefore, biomass-derived carbon nanomaterials should be regarded as multifunctional interfacial platforms in which adsorption, electron transfer, intermediate stabilization and mass transport are coupled within the same nanoarchitecture. This integrated view provides the basis for their use in CO₂ capture, catalytic conversion and electrochemical sensing.

3.1. Ultramicroporosity and Confinement-Enhanced Physisorption

A defining advantage of porous carbon materials is their ability to develop ultramicropores (<1.0 nm), with dimensions approaching the kinetic diameter of CO₂ (~0.33 nm). Within these confined spaces, the adsorption potentials generated by opposing pore walls overlap, creating stronger interaction fields that preferentially stabilize CO₂ over N₂ because of its higher polarizability and quadrupole moment. Ultramicropores in the approximately 0.5–0.7 nm range are particularly effective for enhancing CO₂ uptake at the low partial pressures relevant to post-combustion capture. Accordingly, recent optimization strategies have shifted from maximizing total surface area alone toward controlling the pore-size distribution and increasing the fraction of sub-nanometer pores. This approach can improve low-pressure uptake and CO₂/N₂ selectivity while maintaining moderate adsorption enthalpies that facilitate sorbent regeneration [53,65,67,68].

Recent examples demonstrate how pore architecture and activation conditions translate into improved performance. Biomass-derived carbons produced by hydrothermal carbonization or pyrolysis followed by chemical or physical activation have consistently achieved BET surface areas of ~1500–2300 m² g⁻¹, with pore networks dominated by narrow micropores highly effective for CO₂ capture at relevant partial pressures. A representative example is the synergistic engineering of ultramicroporosity in biomass-derived carbons reported by Cheng et al., where activation strategies generated highly developed micropore networks and surface areas often exceeding 2000 m² g⁻¹, leading to substantial improvements in sorption performance [57].

Crucially, performance is not governed by surface area alone. The distribution of ultramicropores (<0.7 nm), the presence of surface heteroatoms, and surface basicity or acidity jointly modify adsorption energies through mechanisms such as Lewis acid–base interactions. Recent studies on heteroatom-enriched biomass carbons showed that N/S co-doping improved CO₂ adsorption to 4.46 mmol g⁻¹ at 25 °C and 6.38 mmol g⁻¹ at 0 °C and 1 bar, illustrating how controlled doping combined with tailored microporosity enhances capacity under both ambient and sub-ambient conditions [69]. Even higher capacities have been reported for KOH-activated biomass carbons, reaching ~5.95 mmol g⁻¹ at 25 °C and 9.65 mmol g⁻¹ at 0 °C, highlighting the role of extensive ultramicroporosity in maximizing low-temperature sorption while maintaining strong room-temperature performance [38].

For instance, Xing et al. [37] designed an ultramicroporous adsorbent from cotton fibers by adjusting the alkali/carbon ratio and activation temperature, demonstrating that optimization of the sub-nanometer regime can convert common biomass into a competitive sorbent. Likewise, Cheng et al. [57] showed that molten-salt synergistic activation can regulate ultramicroporosity, generate abundant pores below 0.7 nm, and significantly improve CO₂ capture performance, selectivity, diffusion kinetics, and cyclic stability (Figure 3). A further emerging strategy uses hard waste feedstocks to generate selective ultramicroporosity. In recent work, cigarette butts were converted into an N-doped carbon with average pore diameters of 0.6–0.8 nm, identifying this window as particularly effective for CO₂ capture.

3.2. Surface Chemistry: Doping and Heteroatom Availability

While ultramicropore confinement establishes the baseline for physisorption-driven CO₂ capture, surface chemistry plays a decisive role in modulating adsorption strength, selectivity, and moisture tolerance. One of the key advantages of carbon nanomaterials is their ability to undergo controlled heteroatom incorporation (e.g., N, S, O, P) without structural collapse, enabling fine regulation of local electronic density and surface polarity [70,71]. Nitrogen doping alters both the electronic properties and adsorption energetics of carbon nanomaterials, allowing for fine-tuned control over surface polarity and localized electron density [54].

Beyond single dopants, co-doping strategies, such as N/S incorporation, further modulate surface polarity, hydrophilicity, and adsorption microenvironments. By inducing asymmetric charge distribution and localized electric fields, N/S co-doped carbons can enhance quadrupole–field interactions with CO₂ while tuning competitive adsorption in humid streams. Tang et al. [69] reported that N/S co-doped ultramicroporous carbons achieved CO₂ uptakes of 4.46 mmol g⁻¹ at 25 °C and 6.38 mmol g⁻¹ at 0 °C (1 bar), demonstrating the synergistic interaction between heteroatom functionality and pore confinement. Recent studies further confirm the central role of surface chemistry in governing CO₂ interactions. Dziejarski et al. [38] showed that activation strategy controls not only microporosity but also surface functionality, directly influencing CO₂/N₂ selectivity through combined effects of pore-size distribution and adsorption energetics. Similarly, Ji et al. [29] highlighted how nitrogen-coordinated carbon environments stabilize key CO₂ reduction intermediates in electrocatalysis, emphasizing that the electronic perturbations introduced by heteroatom doping can govern both adsorption and catalytic pathways.

Nevertheless, surface functionalization must be carefully optimized. Excessive oxygen-containing groups can increase hydrophilicity and promote competitive H₂O adsorption, reducing effective CO₂ working capacity under humid flue gas conditions. Therefore, the most effective carbon nanomaterials achieve balanced polarity: sufficient to enhance CO₂ affinity through electrostatic and Lewis acid–base interactions, but moderate enough to preserve hydrophobicity and minimize moisture interference. In this framework, surface chemistry is not an auxiliary modification but a primary design variable that acts synergistically with pore confinement to define adsorption thermodynamics, selectivity, and catalytic reactivity [10,55,56].

3.3. Electronic Structure and Catalytic Microenvironments

Beyond adsorption-driven capture, carbon nanomaterials actively participate in CO₂ conversion through their tunable electronic structure and defect chemistry. Unlike inert supports, engineered carbon frameworks can directly influence catalytic performance by redistributing charge density, stabilizing reaction intermediates, and shaping the local chemical environment at the nanoscale. Their catalytic behavior arises from the interplay between graphitic ordering, defect density, and heteroatom incorporation, which together determine electron transport and stabilization of adsorbed species [66,72]. Extended π-conjugated domains provide efficient pathways for electron mobility, a prerequisite for electrochemical and photocatalytic CO₂ reduction. At the same time, structural defects and edge sites introduce localized electronic perturbations that can act as adsorption and activation centers. Nitrogen doping further refines this electronic landscape. Graphitic nitrogen enhances conductivity and promotes charge delocalization within the carbon lattice, while pyridinic nitrogen generates electron-rich regions capable of interacting with CO₂ and stabilizing key intermediates such as COOH and CO. Through these modifications, heteroatom doping does not merely introduce chemical functionality; it redefines the catalytic microenvironment [73,74,75].

The relevance of this electronic tuning is exemplified by biomass-derived N-doped graphitized carbons reported by Hao et al. [17], which achieved 91% Faradaic efficiency toward CO formation during electrochemical CO₂ reduction. In that system, graphitic domains ensured efficient electron transport, while nitrogen-induced active sites modulated intermediate binding energies, illustrating how structural and electronic features cooperate to determine catalytic selectivity. More broadly, recent advances in transition metal-nitrogen-carbon architectures have clarified how atomically dispersed metal centers embedded in doped carbon matrices create well-defined catalytic environments. As discussed by Ji et al. [29], the coordination structure around transition metal nitrogen carbon sites governs adsorption energies, suppresses competing hydrogen evolution, and enhances CO₂ to CO selectivity. Crucially, the surrounding carbon lattice is not a passive host; it actively tunes the electronic structure of the catalytic center through defect-induced polarization and charge redistribution.

This coupling between adsorption energetics and electronic structure highlights a central principle: the same features that enhance CO₂ affinity, including defects, heteroatom-induced polarization, and local electric fields, also determine catalytic pathways. Effective CO₂ conversion requires an optimal binding-energy window, where intermediates are sufficiently stabilized to lower activation barriers but not so strongly bound as to impede product desorption. Carbon nanomaterials are particularly suited to operate within this window because their electronic properties can be continuously modulated through synthesis [45,76,77]. In addition, porous carbon frameworks can locally concentrate CO₂ near active sites, increasing reactant availability and facilitating three-phase interfaces in gas liquid solid electrochemical systems. In photocatalytic assemblies, conductive carbon domains can function as electron reservoirs, improving charge separation and reducing recombination losses. Through this combination of adsorption capability, electronic conductivity, and structural adaptability, carbon nanomaterials provide an integrated platform in which capture and conversion processes are intrinsically coupled rather than sequentially separated [60,61,78].

4. CO₂ as a Target Molecule: Capture, Catalytic Valorization and Electrochemical Detection

CO₂ is both an unavoidable emission from hard-to-abate sectors and a widely available C1 feedstock whose utilization can help close the carbon loop when coupled with low-carbon energy inputs. For this reason, CO₂ management is increasingly framed not as a single-technology challenge, but as an integrated strategy combining efficient separation from dilute streams with conversion pathways capable of producing fuels or commodity chemicals at meaningful rates and selectivities. Recent system-level analyses emphasize that capture, transport, utilization, and storage must be assessed together, because the viability of CO₂ valorization strongly depends on separation cost, stream composition, particularly humidity, and regeneration penalties [79,80].

4.1. CO₂ Capture: From Pore Engineering to Process-Relevant Adsorption Behavior

Carbon dioxide capture refers to the separation and concentration of CO₂ from dilute gas streams before utilization or storage. In industrial practice, capture is the most energy-intensive and cost-determining step in the carbon management chain. Typical post-combustion flue gases from coal-fired power plants contain 13–15% CO₂ on a dry basis, whereas natural gas combined-cycle plants emit 3–5% CO₂. In cement, steel, and waste-to-energy sectors, concentrations vary between 15 and 30%, often in the presence of water vapor, SOx, NOx, and particulates [62,81,82]. Globally, carbon capture facilities currently in operation capture approximately 45–50 Mt CO₂ per year, while climate-neutrality scenarios require capture capacities exceeding several gigatonnes annually by 2050. One of the main barriers remains the energy penalty associated with separation: conventional amine scrubbing typically imposes an efficiency loss of 20–30% in coal power plants, with regeneration energies commonly in the range of 3–4 GJ per tonne of CO₂. Therefore, improving the thermodynamic and kinetic efficiency of capture is essential to reduce overall system cost [63,64,83].

Besides carbon-based adsorbents, several non-carbon nanomaterials, particularly zeolites and metal–organic frameworks (MOFs), have been extensively investigated for CO₂ capture owing to their high surface areas, well-defined pore structures, and excellent adsorption selectivity [19,84]. Nevertheless, many of these materials face challenges related to relatively high synthesis costs and large-scale production and availability. By comparison, biomass-derived carbon materials offer a renewable and cost-effective alternative, combining modular pore structure, good chemical stability, and facile regeneration, making them attractive candidates for practical carbon capture applications.

In this context, porous carbon nanomaterials offer a fundamentally different capture mechanism compared with chemisorption-based systems. CO₂ uptake in carbon frameworks is primarily governed by physisorption enhanced by nanoscale confinement and surface polarization, enabling rapid adsorption/desorption kinetics and lower regeneration penalties when adsorption enthalpies remain within a moderate range (~20–40 kJ mol⁻¹). A representative example is the dynamic gradient porosity engineering approach reported by Shi et al. [85], which describes a strategy for tailoring gradient porosity in biomass-derived porous carbons for efficient CO₂ capture. This work highlights how activation design can create transport-accessible pathways while preserving strong adsorption domains. Humidity tolerance is another decisive industrial parameter. Flue gases typically contain 5–10% water vapor, and competitive adsorption can significantly reduce effective CO₂ working capacity. Unlike strongly hydrophilic chemisorbents, carbon sorbents can be tuned to balance moderate surface polarity, introduced through heteroatom doping, with partial hydrophobicity, preserving performance in wet streams while avoiding the oxidative degradation commonly observed in amine systems [76,86].

Overall, CO₂ capture is not only a materials problem but also a thermodynamic and process-integration challenge. Within this framework, carbon nanomaterials emerge as process-adaptable sorbent platforms. Their performance is governed not by maximum surface area alone, but by the deliberate engineering of adsorption microenvironments compatible with industrial cycling, humidity exposure, and realistic energy constraints. For capture from dilute or mixed-gas streams, the practical objective is not simply to maximize equilibrium uptake at 1 bar, but to achieve high working capacity, fast kinetics, and stability under repeated cycles and realistic gas compositions [45,77,87,88].

4.2. CO₂ Conversion: Nanoengineered Carbon Materials as Supports and Co-Catalysts in Catalytic and Electrochemical Pathways

When shifting from separation to valorization, the function of carbon nanomaterials evolves from passive sorbents to active structural and electronic mediators. In CO₂ conversion processes, CNMs rarely act as the sole catalytic phase; rather, they define how catalytically active metals, oxides, or single-atom sites behave under reaction conditions. Their contribution lies in controlling dispersion, interfacial charge transfer, local reactant concentration, and long-term structural stability [88].

In thermocatalytic hydrogenation pathways, such as CO₂ methanation or reverse water–gas shift, carbon-derived supports offer advantages over conventional oxide supports because their surface chemistry and porosity can be engineered independently of the active phase [39]. Biomass-derived biochars and activated carbons have recently been explored as platforms for Ni-, Co-, and Fe-based methanation catalysts, where high surface area and tailored defect chemistry promote uniform metal nanoparticle dispersion and suppress sintering. One study demonstrated the feasibility of olive-kernel biochar as a support for CO₂ methanation catalysts, achieving 72% CO₂ conversion and 95.5% CH₄ selectivity, while also showing structural stability under prolonged reaction conditions [39]. Similarly, Co nanoparticles supported on engineered biochar achieved up to 60% CO₂ conversion at elevated temperatures, with methane selectivity approaching 80% when promoter elements such as Ce were introduced [40]. These results illustrate how the carbon matrix influences metal–support interactions, modulates interfacial electron density, and governs catalytic selectivity.

The role of carbon nanomaterials becomes even more pronounced in electrochemical CO₂ reduction, where electrical conductivity, mass transport, and interfacial charge distribution are decisive [13]. Carbon nanofibers, graphene-like sheets, and defect-rich porous carbons are widely used as conductive scaffolds that minimize ohmic losses and enable homogeneous current distribution. In a recent study, a self-supporting SnO₂/carbon nanofiber electrode achieved Faradaic efficiencies above 90% toward formate production, demonstrating how embedding active metal oxides within a conductive carbon framework enhances electron transport and stabilizes the catalyst under reductive potentials [41]. In such systems, the carbon matrix is not merely a current collector; it influences intermediate binding energies through electronic coupling and provides hierarchical porosity that facilitates CO₂ diffusion and electrolyte access.

Beyond nanoparticle-supported systems, single-atom catalysts anchored on nitrogen-doped carbon matrices represent a frontier in CO₂ conversion research. Atomically dispersed metal centers, such as Fe–N₄, Co–N₄, and Ni–N₄ motifs embedded in graphitic carbon frameworks, exhibit well-defined coordination environments that enable precise tuning of adsorption energies for key intermediates [89]. The surrounding carbon lattice modulates electron density at the active site through defect-induced polarization and heteroatom doping, suppressing competing hydrogen evolution and enhancing CO or multicarbon product selectivity. In these architectures, the synergy between the metal center and carbon support becomes inseparable; catalytic behavior emerges from the coupled electronic structure of the entire composite rather than from the metal alone.

Photocatalytic CO₂ reduction also benefits from carbon nanomaterial integration. Non-carbon materials, particularly semiconducting metal oxides such as TiO₂ and ZnO, have been widely studied for the photocatalytic reduction of CO₂ due to their intrinsic photoactivity. However, bio-based carbon nanomaterials have not been studied alone and are often integrated with these semiconductors to improve charge separation, electron transport, and catalyst stability, highlighting their complementary rather than competitive role. Indeed, carbon domains can function as electron reservoirs, promoting charge separation when coupled with semiconductors such as TiO₂, CdS, or g-C₃N₄ [90,91]. By acting as conductive bridges, carbon nanostructures reduce electron–hole recombination and facilitate interfacial transfer of photogenerated carriers to adsorbed CO₂ molecules [90]. In these hybrid systems, the adsorption capacity of carbon components can locally enrich CO₂ near active semiconductor sites, increasing effective reaction rates and improving apparent quantum efficiencies.

A recurring theme across thermocatalytic, electrochemical, and photocatalytic routes is that carbon nanomaterials govern active-phase stability under operating conditions. Strong metal–support interactions within defect-rich carbon matrices can inhibit nanoparticle agglomeration and mitigate metal leaching. Moreover, the chemical resistance of graphitic domains improves tolerance to reductive or oxidative environments compared with many oxide supports. These properties are particularly relevant for long-term operation and scale-up.

Importantly, carbon nanomaterials also enable conceptual integration between capture and conversion [92]. Porous carbon supports capable of adsorbing CO₂ can increase the local concentration of reactants at catalytic interfaces by lowering diffusion barriers [15]. This opens the way to dual-function materials in which adsorption and catalytic transformation occur within the same structure, reducing the need for intermediate separation steps. This integration is consistent with emerging strategies focused on reactive capture and process intensification. Overall, carbon nanomaterials contribute to CO₂ conversion not simply by providing surface area, but by defining the electronic, structural, and transport environment in which catalytic events occur. Through controlled defect engineering, heteroatom doping, and hierarchical porosity, CNMs bridge adsorption thermodynamics and reaction kinetics, positioning them as enabling materials for integrated carbon utilization technologies.

Overall, thermocatalytic, electrochemical and photocatalytic CO₂ conversion pathways exploit different functions of carbon nanomaterials: thermal stability and metal dispersion in hydrogenation reactions, conductivity and microenvironment control in electroreduction, and charge separation/local CO₂ enrichment in photocatalysis.

Electrochemical Cell Configurations for CO₂ Conversion

Electrochemical CO₂ conversion is strongly influenced not only by catalyst composition, but also by cell architecture, which governs CO₂ transport, ionic conduction, ohmic resistance, product separation, and overall energy efficiency. H-type cells are widely used for laboratory-scale catalyst screening and mechanistic investigations [93]. In these systems, the cathodic and anodic compartments are separated by an ion-exchange membrane, while CO₂ is dissolved in the catholyte before reaching the electrode surface. Although this configuration allows straightforward operation and independent control of the two electrode compartments, the low solubility and slow diffusion of CO₂ in aqueous electrolytes limit mass transport and achievable current densities.

Flow cells partially overcome these limitations by continuously supplying CO₂ to a gas-diffusion electrode, thereby shortening diffusion pathways and enabling higher reaction rates and current densities. However, their operation requires careful control of electrolyte flow, electrode flooding, gas crossover, carbonate formation, and salt precipitation. Membrane electrode assemblies (MEAs) provide a more compact architecture by integrating catalyst layers with an ion-exchange membrane, reducing ionic transport distances and ohmic losses. In zero-gap electrolyzers, the electrodes are placed in direct contact with the membrane, further minimizing resistance and enabling operation at industrially relevant current densities. Despite these advantages, MEA and zero-gap systems remain affected by membrane degradation, water-management limitations, carbonate crossover, product contamination, and long-term stability issues [94,95]. Quantitatively, many conventional H-type and liquid-electrolyte flow cells operate at current densities below 50 mA cm⁻², whereas current densities of approximately 200 mA cm⁻² or higher are commonly regarded as necessary for process-relevant CO₂ electrolysis. A silver-based gas-diffusion electrolyzer, for example, sustained current densities of up to 300 mA cm⁻² for more than 1200 h of continuous operation [96]. More recently, a 25 cm² zero-gap MEA equipped with a perforated cation-exchange membrane achieved a Faradaic efficiency above 75% for formic acid at 300 mA cm⁻² and a cell voltage below 2 V, while maintaining stable performance for 55 h at 200 mA cm⁻² [97]. These results illustrate how the transition from conventional batch cells to gas-fed and zero-gap architectures can substantially improve reaction rates and move CO₂ electrolysis toward process-relevant operating conditions.

4.3. Electrochemical Pathways for CO₂ Detection

Electrochemical pathways for CO₂ detection have gained increasing relevance because they enable direct transduction of gas-surface interactions into measurable electrical signals through redox reactions, ionic transport, potential shifts, or conductivity changes. Unlike purely optical or thermal sensing platforms, electrochemical sensors provide a flexible framework in which sensitivity and selectivity can be tuned through electrolyte composition, electrode design, and interfacial chemistry [98,99]. Current developments range from high-temperature solid-state devices to room-temperature ionic-liquid-based systems, as well as emerging transistor and metal-organic framework (MOF) platforms, each exploiting a specific electrochemical route for CO₂ recognition and signal generation. Consequently, electrochemical sensing has become one of the most promising strategies for compact, low-power, and application-oriented CO₂ monitoring across environmental, industrial, and indoor scenarios [100,101,102].

Although several of these architectures rely primarily on non-carbon materials, including metal oxides, solid electrolytes, ionic liquids, and MOFs, they are discussed here as comparative benchmarks for evaluating the role, advantages, and limitations of carbon-based and hybrid sensing interfaces. Carbon materials can provide high electrical conductivity, large accessible interfacial areas, chemical functionalizability, and compatibility with miniaturized electrodes. By contrast, non-carbon systems may offer stronger intrinsic CO₂-recognition or ion-conduction mechanisms, although some require elevated operating temperatures or exhibit pronounced sensitivity to humidity and long-term signal drift.

One of the most established electrochemical detection modes is based on redox- or current-driven pathways, particularly in amperometric configurations. In these systems, the analytical response is obtained from a current proportional to the transport or electrochemical conversion of CO₂-related species at the electrode/electrolyte interface. For instance, proton-conducting solid electrolytes operated at 500–600 °C can provide precise CO₂ detection in the 2–14 vol% range in wet N₂ through diffusion-limited current responses, demonstrating the applicability of this approach to industrial gas streams [103]. Likewise, Li-garnet solid electrolytes, such as Li₇La₃Zr₂O₁₂ and its doped variants, have shown fast responses, on the order of one minute, for 400–4000 ppm CO₂ by tailoring electrode electrochemistry to promote CO₂-dependent charge-transfer processes [104]. In aqueous systems, a CuxO/Cu working electrode in NaCl/KCl medium has also been used to quantify CO₂ concentrations up to 8000 ppm by exploiting CO₂-related redox behavior characterized through cyclic voltammetry and Tafel analysis [105]. These examples show that amperometric sensing can operate effectively in both high-temperature solid-state environments and liquid-phase electrochemical systems.

A second important pathway is potentiometric sensing, in which the signal is recorded as an open-circuit voltage associated with CO₂-driven ionic equilibria. Potentiometric CO₂ sensors are especially attractive because of their low power consumption and simple signal acquisition, making them suitable for greenhouse gas monitoring and long-term environmental applications [106,107]. In solid-state systems, the sensing response originates from the interaction of CO₂ with selective electrodes and ionic conductors, generating a measurable electrochemical potential difference. A representative example is the PIM-1/DBU-imidazolate membrane, which exhibits a logarithmic relationship between CO₂ concentration and voltage variation. This behavior has been attributed to reversible CO₂ binding and ion-exchange processes within the membrane, showing how membrane chemistry can directly regulate sensor response. Such potentiometric concepts are particularly relevant for low-energy monitoring in indoor or confined human environments, where continuous operation is required [108].

More recently, conductivity- and impedance-based electrochemical pathways have expanded the field through the incorporation of ionic liquids, MOFs, and hybrid interfaces. Ionic liquids are especially relevant because CO₂ can react with them to form carbamate- or carboxylate-like species, modifying ionic conductivity, viscosity, and the electrical double-layer structure. These physicochemical changes can be transduced through several device architectures, including room-temperature ionic-liquid gels deposited on ITO, where impedance shifts were observed for 500–2000 ppm CO₂ in humid air [108], and microfluidic sensors based on [EMIM][2-CNpyr], in which conductivity or impedance changes arise from CO₂ adduct formation in miniaturized systems [109]. In parallel, MOF-based electrochemical sensing follows a distinct route in which chemisorption directly alters framework transport properties. In CDMOF-2, for example, CO₂ reacts with cyclodextrin hydroxyl groups to form alkyl carbonate species, causing a marked decrease in proton conductivity that can be monitored by impedance spectroscopy [110]. These results demonstrate that electrochemical CO₂ detection can be achieved not only through electron-transfer reactions, but also through controlled perturbations of ionic transport and interfacial conductivity.

Electrolyte-gated transistors (EGTs) represent another emerging class of electrochemical CO₂ sensors, in which the sensing mechanism is coupled to semiconductor channel modulation. In₂O₃/ionic-liquid EGTs have shown operation under ambient conditions in the 400–4000 ppm CO₂ range through a pseudo-gating mechanism involving the combined action of CO₂, H₂O, and O₂ near the semiconductor interface. These reactions alter the local concentration of oxygen species and modify channel conductivity. Importantly, impedance spectroscopy has proven useful for identifying mixed ionic/electronic conduction regimes in these devices, revealing that the strongest CO₂ response is often found in frequency domains dominated by metal-oxide-related processes rather than purely ionic ones [111,112]. This confirms that combining direct-current measurements with electrochemical impedance analysis is increasingly necessary to distinguish true CO₂ responses from overlapping interfacial or humidity-related effects. Although these sensing architectures differ in operating temperature, electrolyte configuration, electrode design and signal output, they share a common principle: CO₂ recognition is converted into a measurable electrical response through interfacial redox processes, ionic equilibria, conductivity changes or semiconductor channel modulation. Amperometric sensors rely on current variations associated with CO₂-related electrochemical reactions or diffusion-limited transport, whereas potentiometric devices convert CO₂-driven ionic equilibria into open-circuit voltage shifts. Impedance- and conductometric sensors detect changes in ionic transport, double-layer structure or framework conductivity, particularly in ionic-liquid, MOF and hybrid interfaces. Electrolyte-gated transistor platforms further extend this concept by coupling CO₂-dependent interfacial chemistry with modulation of the semiconductor channel. Therefore, electrochemical CO₂ sensing should be viewed as a family of interfacial transduction strategies rather than a single detection mechanism. The main electrochemical transduction pathways for CO₂ detection are summarized in Figure 4.

A comparative overview of the main electrochemical CO₂ sensing architectures, transduction mechanisms and critical limitations is provided in

Table 3. Electrochemical CO₂ sensing architectures and transduction mechanisms.

Sensor Type	Representative Interface/Material	Main Transduction Mechanism	Signal Output	Operating Detection Range Reported in Text	Main Advantages	Critical Limitations	Ref.
Amperometric CO2 sensor	Proton-conducting solid electrolyte	CO2-related diffusion-limited current response at high temperature	Current variation	2–14 vol% CO2 in wet N2; 500–600 °C	Suitable for industrial gas streams; direct current readout	High operating temperature; humidity and gas-composition dependence	[103]
Amperometric/solid-state sensor	Li-garnet solid electrolyte, e.g., Li7La3Zr2O12-based systems	CO2-dependent charge-transfer processes at tailored electrodes	Current response/electrochemical signal	400–4000 ppm CO2; response on the order of ~1 min	Fast response; solid-state configuration	Electrode/electrolyte engineering required; possible cross-sensitivity	[104]
Aqueous electrochemical CO2 sensor	CuxO/Cu working electrode in NaCl/KCl medium	CO2-related redox behavior characterized by CV and Tafel analysis	Current response	Up to 8000 ppm CO2	Simple aqueous configuration; redox-based quantification	Electrolyte composition and electrode stability may affect reproducibility	[105]
Solid-state potentiometric CO2 sensor	Selective electrodes coupled with ionic conductors	CO2 interaction with electrode/electrolyte generates electrochemical potential difference	Voltage response	Application-dependent	Simple signal acquisition; low power demand	Temperature dependence; electrode selectivity; long-term drift	[106,107]
Potentiometric CO2 sensor	PIM-1/DBU-imidazolate membrane	Reversible CO2 binding and ion-exchange equilibria	Open-circuit voltage shift	Logarithmic voltage response versus CO2 concentration	Low-power operation; suitable for continuous monitoring	Membrane stability, humidity effects and drift must be controlled	[108]
Impedance-based CO2 sensor	Room-temperature ionic-liquid gel on ITO	CO2-induced changes in ionic conductivity and electrical double layer	Impedance variation/ΔZ	500–2000 ppm CO2 in humid air	Room-temperature operation; sensitive to interfacial ionic changes	Humidity cross-sensitivity; response/recovery and drift	[113]
Microfluidic ionic-liquid CO2 sensor	[EMIM][2-CNpyr]-based microfluidic interface	CO2 adduct formation modifies conductivity/impedance	Conductivity or impedance change	Application-dependent	Miniaturization potential; tunable ionic-liquid chemistry	Viscosity, humidity and long-term ionic-liquid stability	[109]
MOF-based impedance sensor	CDMOF-2	CO2 reaction with cyclodextrin hydroxyl groups forming alkyl carbonate species	Proton conductivity decrease/impedance shift	Application-dependent	Chemisorption-driven selectivity; framework-based transduction	Framework stability, reversibility and moisture sensitivity	[110]
Electrolyte-gated transistor CO2 sensor	In2O3/ionic-liquid electrolyte-gated transistor	Pseudo-gating mechanism involving CO2, H2O and O2 near semiconductor interface	Semiconductor channel current modulation	400–4000 ppm CO2 under ambient conditions	Ambient operation; amplified transistor response	Mixed ionic/electronic effects; humidity and oxygen interference	[111]
Electrolyte-gated transistor/impedance-assisted platform	In2O3/ionic-liquid hybrid interface	Frequency-dependent mixed ionic/electronic response; CO2-sensitive interfacial processes	Channel current + impedance response	Application-dependent	EIS helps separate CO2 response from overlapping interfacial effects	Requires careful interpretation of frequency domains and humidity effects	[112]
General electrochemical CO2 sensing platforms	Amperometric, potentiometric, impedance/conductometric and transistor-based systems	Redox reactions, ionic equilibria, adduct formation, transport modulation and gating	Current, voltage, impedance/conductivity or channel current	From ppm to vol% depending on architecture	Flexible design; suitable for compact and low-power monitoring	Humidity, O2 interference, temperature dependence, drift and long-term stability	[98,100,114]

. The table summarizes representative electrochemical pathways for CO₂ detection, including amperometric, potentiometric, impedance/conductometric and electrolyte-gated transistor configurations. These devices convert CO₂ recognition into measurable current, voltage, impedance/conductivity or channel-current signals through interfacial redox reactions, ionic equilibria, adduct formation, transport modulation or semiconductor channel gating. Key challenges include humidity cross-sensitivity, oxygen interference, temperature dependence, response/recovery behavior, drift and long-term operational stability.

Despite considerable progress, several challenges continue to limit the broader implementation of electrochemical CO₂ sensors. The most significant include temperature dependence, long-term operational stability, and cross-sensitivity to humidity and oxygen, especially in systems where ion transport and reactive interfacial processes dominate the signal [114,115]. Current optimization strategies therefore focus on tailoring ionic-liquid composition, developing more robust solid electrolytes, refining internal sensor architectures, and using impedance spectroscopy to decouple the specific contribution of CO₂ from those of water vapor and other interfering gases [116,117]. Overall, the literature indicates that potentiometric, amperometric, and resistive/impedance-based architectures constitute the principal electrochemical routes for CO₂ detection, each offering a different balance between sensitivity, selectivity, operating temperature, and miniaturization potential. Continued advances in materials design and electrochemical interface engineering are expected to support the development of more stable, low-power, and highly sensitive CO₂ sensors for industrial and environmental monitoring [118,119].

These limitations indicate that future progress will depend not only on material innovation, but also on the alignment between material properties and process-level requirements.

5. Challenges, Scale-Up Limitations and Future Industrial Translation

Despite substantial progress in the rational design of biomass-derived carbon nanomaterials for CO₂ capture, adsorption, sensing, and catalytic valorization, their translation from proof-of-concept studies to technologically relevant systems remains constrained by several scientific and engineering barriers (Figure 5). A recurring limitation is that most reported performances are obtained under idealized laboratory conditions, often using pure CO₂ streams, simplified binary mixtures, short testing periods, or static adsorption configurations [25,120]. However, industrial CO₂-containing streams are compositionally complex and may include water vapor, O₂, SO_x, NO_x, volatile organics, particulate matter, and fluctuating temperature and pressure. These factors can significantly alter adsorption thermodynamics, competitive uptake, surface chemistry, pore accessibility, and catalyst stability. Therefore, future studies should move beyond equilibrium adsorption capacity at 1 bar as the dominant performance descriptor and instead prioritize working capacity, breakthrough behavior, adsorption/desorption kinetics, regeneration energy, cyclic stability, and tolerance to realistic contaminants. This shift is essential because industrial carbon capture is governed not only by maximum uptake, but by the ability of a material to operate repeatedly, selectively, and efficiently under dynamic flow conditions [121,122,123].

A second major challenge is associated with the intrinsic variability of biomass-derived precursors and the reproducibility of carbon nanomaterial synthesis. Although biomass offers clear advantages in renewability, abundance, and circularity, its chemical composition depends strongly on botanical origin, geographic location, harvesting conditions, storage, moisture content, lignocellulosic composition, ash fraction, and mineral impurities. These variables influence carbon yield, graphitization degree, heteroatom retention, surface oxygen functionality, pore-size distribution, and electrical conductivity [23,24].Consequently, two materials prepared under nominally identical pyrolysis or activation conditions may exhibit substantially different physicochemical properties and, therefore, different adsorption or catalytic performances. This limitation is particularly relevant for highly engineered materials whose activity depends on narrow ultramicropore windows, specific nitrogen configurations, controlled defect densities, or atomically dispersed catalytic sites [124,125]. Industrial translation will therefore require robust precursor standardization, pre-treatment protocols, quality-control criteria, and scalable synthesis routes capable of producing reproducible structure–function relationships. Without this level of process control, performance advantages demonstrated at laboratory scale may be difficult to reproduce in pilot or full-scale systems [94,126].

From a sustainability and economic perspective, the classification of biomass-derived carbons as “green” or “low-cost” materials must be treated with caution. While the renewable origin of the precursor is advantageous, the overall environmental and economic profile may be dominated by activation chemistry, thermal energy demand, washing steps, reagent recovery, solvent use, wastewater generation, and post-synthetic functionalization [24,127]. Chemical activation with KOH, H₃PO₄, ZnCl₂, molten salts, or nitrogen-rich additives can generate highly developed porosity and excellent CO₂ uptake; however, these benefits are often accompanied by substantial reagent consumption, corrosion issues, purification requirements, and secondary waste streams. For this reason, recent life-cycle and techno-economic studies increasingly emphasize that the environmental value of waste-derived activated carbons must be evaluated through integrated metrics, including carbon yield, activator-to-precursor ratio, reagent recovery efficiency, regeneration energy, sorbent lifetime, and CO₂ captured per unit of material produced [128,129,130]. In this context, life-cycle assessment and techno-economic assessment should not be considered complementary analyses at the end of material development, but decision-making tools integrated from the earliest stages of synthesis design.

For CO₂ conversion technologies, additional barriers arise from the complexity of catalytic interfaces and the demanding stability requirements of continuous operation. Carbon nanomaterials can enhance CO₂ electroreduction and photoreduction by improving conductivity, stabilizing active sites, modulating intermediate binding energies, and facilitating interfacial charge transfer [131]. However, many reported systems are still evaluated over short reaction times and under highly controlled conditions that do not capture long-term deactivation phenomena [132]. In electrochemical CO₂ reduction, carbon-based catalysts and supports may suffer from flooding, salt precipitation, carbon corrosion, metal migration, nanoparticle agglomeration, restructuring of single-atom sites, or changes in local pH and electrolyte composition. In photocatalytic systems, low quantum efficiency, charge recombination, photocorrosion, insufficient visible-light harvesting, and catalyst recovery remain major obstacles [133,134]. Moreover, selectivity is highly sensitive to local reaction microenvironments, including CO₂ concentration near the active site, proton availability, electrolyte identity, hydrophobicity, and mass-transfer limitations. Therefore, future research should place greater emphasis on operando characterization, long-duration testing, degradation pathway analysis, and standardized performance metrics, including current density, Faradaic efficiency, product formation rate, energy efficiency, carbon balance, and operational lifetime [93,95,135].

A further limitation concerns the integration of capture and conversion into unified process architectures. In most current configurations, CO₂ capture and catalytic transformation are treated as separate sequential operations, requiring intermediate desorption, compression, purification, transport, or re-feeding into a second reactor. This separation increases energy demand and process complexity, reducing the overall efficiency of carbon utilization [136,137,138]. Biomass-derived carbon nanomaterials offer an attractive platform for integrated capture-conversion systems because their porous structure can concentrate CO₂ near catalytic centers, while their conductive or defect-rich domains can promote electron transfer and intermediate stabilization. Nevertheless, designing dual-function materials remains scientifically challenging because the optimal conditions for adsorption are not necessarily compatible with catalytic turnover [24,139,140]. Strong CO₂ binding may improve capture but hinder desorption or conversion, whereas weak adsorption may limit local reactant availability. Likewise, hydrophilic surface functionalities may enhance interaction with CO₂-derived species but increase competitive water adsorption and reduce stability under humid conditions. Thus, the development of integrated carbon-based platforms requires careful balancing of adsorption enthalpy, pore accessibility, catalytic site density, hydrophobicity, electronic conductivity, and reactor-level mass transport [141,142].

Finally, industrial translation will require closer convergence between materials chemistry, reactor engineering, process modeling, and data-driven optimization. The next generation of carbon nanomaterials should not be designed solely to maximize isolated laboratory metrics, but to satisfy process-level requirements such as pressure drop, pelletization or shaping, mechanical stability, thermal management, regeneration compatibility, synthesis scalability, and integration with existing industrial infrastructure [143,144]. In this regard, artificial intelligence, machine learning, high-throughputexperimentation, and multiscale modeling can accelerate the identification of correlations among precursor chemistry, synthesis parameters, pore architecture, surface functionality, and final performance. However, such approaches will only be reliable if trained on standardized, high-quality datasets that include negative results, realistic testing conditions, and durability information [145,146,147].Therefore, the future industrial relevance of biomass-derived carbon nanomaterials will depend on moving from material discovery toward system-oriented development, where performance, sustainability, cost, durability, and manufacturability are evaluated simultaneously. This transition will be essential for positioning carbon nanomaterials as credible enabling platforms for circular carbon management and low-carbon chemical technologies [24,128,147].

6. Future Perspectives and Conclusions

Future progress in biomass-derived nanoengineered carbon materials will first require a clearer and more standardized classification of the materials under investigation. Carbon nanomaterials with nanoscale external dimensions, such as carbon dots, carbon nanotubes, carbon nanofibers, and graphene-based structures, should be distinguished from biochars, hydrochars, activated carbons, and other porous carbonaceous materials. Hybrid and composite systems should also be identified according to the nature and function of each component. Beyond terminology, standardized reporting protocols are needed to enable meaningful comparisons among studies. These should include the biomass precursor composition, carbon yield, activation conditions, pore-size distribution, surface chemistry, heteroatom speciation, electrical conductivity, material stability, and overall mass balance. Reporting adsorption capacity or catalytic selectivity without information on synthesis yield, reagent consumption, and material recovery can lead to incomplete or misleading assessments of process performance.

A second priority is the transition from idealized laboratory measurements to validation under realistic operating conditions. CO₂-capture studies should increasingly employ humid and multicomponent gas streams containing representative concentrations of N₂, O₂, SO_x, NO_x, and other potential contaminants. Equilibrium uptake measurements should be complemented by dynamic breakthrough experiments, working-capacity evaluations, adsorption–desorption kinetics, regeneration-energy estimates, and extended cyclic testing. Likewise, catalytic and electrochemical studies should move beyond short-duration experiments and assess performance under continuous operation, technologically relevant current densities, realistic reactant concentrations, and complete carbon balances. Long-term tests are particularly important for identifying progressive pore blockage, loss of surface functionalities, catalyst restructuring, electrode flooding, salt precipitation, and other degradation mechanisms that may not be apparent during experiments lasting only a few hours.

Scalable manufacturing with a lower environmental cargo represents a third essential research direction. Although biomass is renewable, the sustainability of the resulting carbon material can be compromised by severe activation conditions, high activator-to-precursor ratios, prolonged thermal treatments, extensive washing, solvent consumption, and secondary wastewater generation. Future synthesis strategies should therefore aim to reduce chemical and energy inputs while preserving control over porosity, surface functionality, and electrical properties. Particular attention should be given to mild or self-activation approaches, physical activation, solvent-free processing, heat integration, water recirculation, and the recovery and reuse of activating agents. Carbon yield and resource efficiency should be considered alongside functional performance, because a material exhibiting exceptional adsorption or catalytic activity may still be environmentally and economically unattractive if its production requires excessive burn-off or purification. The translation of these materials into process-relevant devices will also require closer integration between material design and equipment engineering. Most studies evaluate powdered materials under conditions that do not account for pressure drop, mechanical attrition, heat transfer, or handling requirements. Future research should therefore address pelletization, granulation, monolith formation, coating, and incorporation into structured porous supports while determining how these shaping procedures affect pore accessibility and active-site availability. For electrochemical CO₂ conversion, carbon materials should be engineered directly within gas-diffusion electrodes, membrane electrode assemblies, rather than assessed exclusively in conventional H-type cells. Similarly, capture and thermocatalytic processes should be validated in fixed-bed, fluidized-bed, or continuous-flow configurations. Establishing the relationship between nanoscale material properties and reactor-scale transport phenomena will be necessary to convert promising laboratory materials into practical sorbents, catalysts, and sensing interfaces.

Finally, prospective life-cycle assessment (LCA), techno-economic assessment (TEA), and data-driven design should be incorporated from the earliest stages of material development. LCA and TEA should not be applied only after synthesis optimization, because early-stage assessments can identify environmental and economic hotspots before energy- or reagent-intensive procedures become embedded in the production route. These analyses should use transparent system boundaries and functional bases that account for material lifetime, regeneration, working capacity, product yield, and end-of-life management. At the same time, standardized and openly accessible datasets could support machine-learning approaches capable of linking biomass composition and processing conditions to pore architecture, surface functionality, and application performance. However, reliable predictive models will require consistent metadata, uncertainty reporting, negative results, and data obtained under realistic operating conditions rather than datasets dominated by isolated record values.

Overall, biomass-derived nanoengineered carbon materials comprise both dimensionally nanoscale carbons and porous carbonaceous materials whose functional architecture is deliberately controlled at the nanometer scale. Their complementary properties enable applications ranging from pollutant removal and CO₂ adsorption to catalytic conversion and sensing. However, their future relevance will depend less on isolated record performances than on reproducible synthesis, realistic validation, long-term durability, low-impact manufacturing, and integration into process-relevant devices. By connecting nanoscale engineering with standardized evaluation, prospective sustainability assessment, techno-economic considerations, and reactor-level requirements, this review provides a unified structure–property–performance framework that may guide the rational design and practical implementation of next-generation biomass-derived carbon materials for circular carbon management and low-carbon technologies.