Thermoelectric materials convert heat into electricity through the Seebeck effect and vice versa via the Peltier effect, offering potential for waste heat recovery and solid-state cooling. Carbon-based nanomaterials, particularly graphene and carbon nanotubes (CNTs), have emerged as promising candidates for room-temperature thermoelectric applications due to their unique electronic properties, tunable conductivity, and low-dimensional structure. Their high carrier mobility, flexibility, and compatibility with organic and inorganic matrices make them suitable for integration into flexible electronics, wearable devices, and energy-efficient systems.

The thermoelectric performance of a material is quantified by the dimensionless figure of merit, ZT = (S²σT)/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity. Carbon nanomaterials exhibit high σ and low κ, but their intrinsically low S limits ZT. Strategies such as heteroatom doping, nanostructuring, and forming nanocomposites have been employed to enhance their thermoelectric properties.

Graphene, a single layer of sp²-bonded carbon atoms, possesses exceptional electrical conductivity (up to 10⁶ S/m) and high thermal conductivity (~5000 W/mK). However, its near-zero bandgap and symmetrical electron-hole conduction result in a low Seebeck coefficient (~80 μV/K). To improve S, researchers have introduced heteroatoms like nitrogen (N) and boron (B) into the graphene lattice. N-doping introduces electron donors, shifting the Fermi level into the conduction band, while B-doping creates hole carriers, moving the Fermi level toward the valence band. Controlled doping can optimize the trade-off between σ and S. For instance, N-doped graphene films have demonstrated a Seebeck coefficient of -130 μV/K at room temperature, while B-doped graphene achieves +100 μV/K. Dual doping with both N and B can further modify the electronic structure, creating localized states that enhance phonon scattering and reduce κ without severely compromising σ.

Carbon nanotubes exhibit anisotropic thermoelectric properties depending on their chirality (metallic or semiconducting). Semiconducting CNTs generally show higher S compared to metallic CNTs due to their bandgap. Similar to graphene, doping CNTs with heteroatoms can tune their electronic properties. N-doped CNTs exhibit n-type behavior with S reaching -150 μV/K, while B-doped CNTs display p-type characteristics with S around +120 μV/K. The one-dimensional confinement in CNTs also enhances phonon scattering, reducing lattice thermal conductivity to values as low as 30-50 W/mK. Aligned CNT films or fibers have shown improved thermoelectric performance by leveraging directional charge transport and suppressed cross-plane thermal conduction.

Nanocomposites combining carbon nanomaterials with polymers or metals offer synergistic effects for thermoelectric enhancement. Conductive polymers such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) are often paired with graphene or CNTs to leverage the high σ of carbon materials and the moderate S of polymers. For example, PEDOT:PSS/graphene composites achieve a power factor (S²σ) of 200-300 μW/mK² at room temperature by balancing carrier mobility and energy filtering at interfaces. Similarly, CNT/polyaniline composites exhibit ZT values of 0.1-0.2 due to interfacial phonon scattering and charge carrier energy barriers.

Incorporating metallic nanoparticles (e.g., Au, Ag) into carbon-based composites can further enhance thermoelectric performance through plasmonic effects or energy-dependent scattering. For instance, Au nanoparticle-decorated graphene sheets show a 30-50% increase in S due to selective carrier filtering at the metal-carbon interface. Hybrid systems of CNTs with Bi₂Te₃ nanoparticles have also demonstrated improved ZT by combining the low κ of CNTs with the high S of Bi₂Te₃.

Room-temperature applications of carbon-based thermoelectric nanomaterials include wearable energy harvesters, flexible cooling devices, and self-powered sensors. Thin-film thermoelectric generators using doped graphene or CNT networks can generate microwatts of power from body heat, suitable for powering low-energy electronics. Flexible cooling patches based on CNT-polymer composites have been explored for localized thermal management in medical or electronic devices. Additionally, carbon nanomaterial-based thermosensors exhibit high sensitivity to temperature gradients, enabling precise thermal mapping in microelectronic circuits.

Challenges remain in achieving higher ZT values in carbon-based systems, primarily due to the difficulty in decoupling electrical and thermal transport properties. Future research may focus on advanced doping techniques, interface engineering in nanocomposites, and exploiting quantum confinement effects in low-dimensional carbon structures. Scalable fabrication methods such as roll-to-roll printing or inkjet coating will be critical for commercializing these materials in practical applications.

In summary, carbon-based thermoelectric nanomaterials offer a versatile platform for room-temperature energy conversion and thermal management. Through strategic doping, nanocomposite design, and nanostructuring, their performance can be tailored to meet the demands of next-generation flexible and sustainable thermoelectric devices. Continued advancements in material synthesis and device integration will further unlock their potential in diverse applications.