Three-dimensional graphene aerogels and foams represent an important class of porous nanomaterials with interconnected networks, high surface area, and tunable properties. Their fabrication methods significantly influence pore architecture, mechanical robustness, and electrical conductivity, making them suitable for energy storage and environmental applications. The primary synthesis routes include freeze-drying, chemical reduction, and template-assisted methods, each offering distinct advantages in controlling structural parameters.

Freeze-drying, or lyophilization, is a widely used technique for producing lightweight graphene aerogels with hierarchical porosity. In this process, an aqueous dispersion of graphene oxide (GO) is frozen, causing ice crystals to template the pore structure. Subsequent sublimation under vacuum leaves behind a porous graphene network. The freezing rate determines pore size distribution: rapid freezing produces smaller, more uniform pores, while slow freezing yields larger, anisotropic channels. Mechanical properties are closely tied to this architecture, with compressive strengths typically ranging from 10 kPa to 1 MPa, depending on density and crosslinking. Electrical conductivity varies between 1 and 100 S/m, influenced by the reduction efficiency and contact points between graphene sheets. A key challenge lies in balancing density and porosity—higher graphene concentrations improve mechanical stability but reduce pore volume, impacting surface area and mass transport.

Chemical reduction methods involve the gelation of GO followed by reduction to restore sp2 carbon networks. Hydrazine, ascorbic acid, or hydrothermal treatments are common reducing agents. During gelation, GO sheets crosslink via π-π stacking and residual oxygen groups, forming a 3D network. Pore sizes range from nanometers to micrometers, with surface areas exceeding 500 m²/g after reduction. The mechanical properties of chemically reduced aerogels are often brittle, with elastic moduli in the 1-10 kPa range unless reinforced. However, their electrical conductivity can reach 100-500 S/m due to better restoration of the graphene lattice. Controlling the reduction kinetics is critical—too rapid reduction causes inhomogeneous shrinkage, while slow processes improve pore uniformity but may introduce structural defects.

Template-assisted fabrication employs sacrificial scaffolds, such as polymer foams or metal foams, to dictate the final pore structure. Graphene or GO is coated onto the template, which is later removed via calcination or etching. This method offers precise control over pore size and shape, producing monodisperse pores from 100 nm to several micrometers. The resulting materials exhibit higher mechanical stability, with compressive strengths up to 10 MPa, owing to the ordered architecture. Conductivity values surpass 500 S/m due to continuous graphene pathways. However, template removal can introduce cracks or incomplete replication, and the process is less scalable than freeze-drying or chemical methods.

Pore structure control is essential for optimizing performance in energy storage applications. In supercapacitors, mesopores (2-50 nm) facilitate ion transport, while micropores (<2 nm) increase charge storage via electric double-layer formation. Aerogels with bimodal pore distributions achieve specific capacitances exceeding 300 F/g in aqueous electrolytes. For lithium-sulfur batteries, macropores (>50 nm) accommodate sulfur loading and buffer volume changes, while smaller pores trap polysulfides. Freeze-dried graphene foams in these systems demonstrate sulfur loadings above 5 mg/cm² with cycling stability over 500 cycles.

Environmental applications leverage the high surface area and adsorption capacity of 3D graphene networks. In water purification, functionalized aerogels remove heavy metals like lead and cadmium with capacities exceeding 200 mg/g. The interconnected pores enable rapid diffusion, while oxygen groups on reduced GO enhance binding sites. For oil spill remediation, hydrophobic graphene foams absorb oils selectively, with uptake capacities reaching 50 times their weight. Pore size must be optimized to balance absorption kinetics and capacity—larger pores improve uptake rates but reduce surface area.

Mechanical integrity remains a challenge, particularly in wet or cyclic loading conditions. Crosslinking with polymers or carbon nanotubes can enhance elasticity but often at the expense of conductivity or porosity. Another trade-off arises between density and performance—ultralight aerogels (<10 mg/cm³) offer high surface area but collapse under minimal stress, while denser foams sacrifice porosity for strength.

Conductive network formation depends on the continuity of graphene sheets and defect density. Annealing at high temperatures improves crystallinity but may collapse pores. Doping with nitrogen or sulfur enhances conductivity without compromising structure, with reported values up to 1000 S/m in doped aerogels.

Scalability and cost are barriers to commercialization. Freeze-drying is energy-intensive, chemical reduction uses hazardous agents, and template methods lack economy for mass production. Recent advances focus on ambient-pressure drying and bio-based reductants to address these issues.

In summary, 3D graphene aerogels and foams exhibit tunable pore structures, moderate-to-high conductivity, and adaptable mechanical properties through careful selection of fabrication methods. Their performance in energy storage and environmental applications hinges on balancing porosity, density, and structural stability. Continued research aims to overcome synthesis challenges while expanding their functional versatility.