Carbon aerogels are highly porous, lightweight materials with exceptional properties such as high surface area, electrical conductivity, and thermal stability. Their performance can be further enhanced through chemical and physical functionalization strategies, including doping, surface modification, and heteroatom incorporation. These approaches tailor the aerogels' properties for specific applications, such as energy storage, catalysis, and environmental remediation, by modifying electrical conductivity, wettability, and catalytic activity.

Doping carbon aerogels with heteroatoms like nitrogen or sulfur introduces electron-rich sites that alter electronic properties. Nitrogen doping, for instance, is achieved through methods such as pyrolysis of nitrogen-rich precursors like melamine or urea, or post-treatment with ammonia. The incorporation of nitrogen creates pyridinic, pyrrolic, and graphitic nitrogen configurations, each influencing conductivity differently. Pyridinic nitrogen enhances electron donor capabilities, improving charge transfer in electrocatalytic reactions. Studies show nitrogen-doped carbon aerogels exhibit increased electrical conductivity, with some reaching 250 S/m compared to undoped aerogels at 100 S/m. Sulfur doping, often introduced via thiourea or sulfuric acid treatment, introduces thiophene-like structures that modify surface chemistry. Sulfur-doped aerogels demonstrate improved catalytic activity for oxygen reduction reactions due to the altered charge distribution.

Surface modification techniques such as plasma treatment introduce functional groups without altering bulk properties. Oxygen plasma treatment grafts carboxyl and hydroxyl groups onto the aerogel surface, increasing hydrophilicity. Water contact angles can decrease from 140° to 30° after treatment, making the material suitable for aqueous applications. Conversely, fluorine plasma treatment creates a superhydrophobic surface with contact angles exceeding 150°, useful for oil-water separation. Plasma treatments also affect conductivity; oxygen plasma may reduce it due to disrupted sp² carbon networks, while nitrogen plasma can enhance it by introducing conductive nitrogen-containing groups.

Incorporating heteroatoms like boron or phosphorus modifies the aerogel’s electronic structure. Boron doping, achieved via boric acid or boron oxide, creates electron-deficient sites, improving catalytic performance for oxidative reactions. Boron-doped carbon aerogels show enhanced activity in hydrogen peroxide generation, with Faradaic efficiencies reaching 85%. Phosphorus doping, using phosphoric acid or triphenylphosphine, introduces lone pairs that alter charge distribution. Phosphorus-doped aerogels exhibit improved lithium-ion storage capacity due to additional active sites for ion adsorption.

Chemical functionalization via covalent bonding attaches specific functional groups to the carbon lattice. Oxidation with nitric acid or hydrogen peroxide introduces carboxyl, epoxy, and ketone groups, increasing hydrophilicity and enabling further conjugation with biomolecules or metal nanoparticles. Reduction with hydrazine or thermal annealing restores conductivity by removing oxygenated groups. Grafting polymers like polyethyleneimine through carbodiimide chemistry introduces amine groups, useful for gas adsorption or acid-catalyzed reactions.

Non-covalent functionalization involves π-π stacking or van der Waals interactions with aromatic molecules like pyrene derivatives. This approach preserves the carbon lattice while adding functionalities such as fluorescence or enhanced electrochemical activity. For example, pyrenebutyric acid-functionalized carbon aerogels show improved capacitance in supercapacitors due to additional pseudocapacitive sites.

The choice of functionalization method depends on the desired application. For electrocatalysis, nitrogen doping or metal nanoparticle incorporation is preferred. In environmental applications, oxygen plasma treatment or polymer grafting enhances adsorption capacity. Each method has trade-offs; doping may improve conductivity but reduce porosity, while plasma treatment modifies surface properties without affecting bulk conductivity.

Specific examples illustrate these effects. Nitrogen-doped carbon aerogels used in supercapacitors achieve specific capacitances of 350 F/g, compared to 200 F/g for undoped versions. Sulfur-doped aerogels exhibit improved catalytic activity for CO₂ reduction, with methanol selectivity reaching 60%. Oxygen plasma-treated aerogels show enhanced dye adsorption capacities, removing 95% of methylene blue within 30 minutes. Boron-doped aerogels demonstrate stable performance in fuel cells, with power densities of 800 mW/cm².

Functionalization also impacts mechanical properties. Nitrogen doping can increase Young’s modulus by 20% due to crosslinking effects, while oxidation may reduce it by 30% due to structural defects. Thermal stability varies; phosphorus-doped aerogels retain structure up to 800°C, whereas heavily oxidized aerogels degrade at 400°C.

Future directions include multi-heteroatom doping to synergize effects, such as nitrogen-sulfur co-doping for enhanced catalytic activity. Advanced characterization techniques like X-ray absorption spectroscopy can elucidate atomic-level changes. Scalable methods like microwave-assisted doping or continuous plasma reactors are being explored for industrial applications.

In summary, chemical and physical functionalization of carbon aerogels enables precise control over their properties. Doping, surface modification, and heteroatom incorporation each offer distinct advantages, whether for improving conductivity, wettability, or catalytic performance. These strategies expand the applicability of carbon aerogels across diverse fields, from energy storage to environmental remediation, without relying on composite formation or biomedical uses. The selection of functionalization routes must balance performance enhancements with potential trade-offs in porosity, mechanical strength, or stability.