Few-layer black phosphorus (BP) has emerged as a promising electrode material for supercapacitors due to its unique structural and electronic properties. Unlike conventional carbon-based materials, BP exhibits a puckered layered structure with strong in-plane anisotropy, high theoretical specific capacitance, and tunable electronic properties. The material's ability to intercalate ions efficiently while maintaining structural integrity makes it particularly suitable for energy storage applications. However, challenges related to ambient instability and scalable production must be addressed to realize its full potential.

Liquid-phase exfoliation is the most widely used method for producing few-layer BP. This technique involves dispersing bulk BP crystals in an appropriate solvent and applying ultrasonic energy to delaminate the layers. The choice of solvent is critical to achieving stable dispersions while minimizing degradation. Polar solvents such as N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) are commonly used due to their ability to match the surface energy of BP, facilitating exfoliation. Recent studies have demonstrated that solvents with dissolved oxygen scavengers, such as choline chloride-urea mixtures, can further suppress oxidation during exfoliation. The concentration of BP, sonication time, and centrifugation parameters must be optimized to yield few-layer flakes with minimal defects.

Stabilization of exfoliated BP remains a significant challenge due to its susceptibility to oxidation under ambient conditions. Several strategies have been developed to enhance stability, including surface passivation and covalent functionalization. Encapsulation with inert polymers like poly(methyl methacrylate) (PMMA) or hexagonal boron nitride (h-BN) has been shown to slow degradation. Covalent functionalization, particularly through aryl diazonium chemistry, introduces stable organic groups that passivate reactive lone pairs on phosphorus atoms. For instance, nitrophenyl-functionalized BP exhibits extended stability in air while retaining electrochemical activity. Additionally, in-situ polymerization of conductive polymers such as polyaniline (PANI) on BP surfaces provides both stabilization and improved charge transport.

The anisotropic charge transport properties of BP significantly influence its performance as a supercapacitor electrode. The puckered structure results in different effective masses for charge carriers along the armchair and zigzag directions, leading to direction-dependent conductivity. Measurements have shown that the armchair direction exhibits higher carrier mobility, with reported values exceeding 1,000 cm² V⁻¹ s⁻¹ in few-layer samples. This anisotropy must be considered in electrode design to maximize charge collection efficiency. Aligning BP flakes along the preferred conduction direction or using conductive additives like carbon nanotubes can mitigate resistive losses.

Voltage-dependent capacitance is another distinctive feature of BP-based supercapacitors. Unlike conventional materials with near-ideal double-layer behavior, BP exhibits a pronounced voltage-dependent response due to its semiconducting nature and Faradaic processes. At low voltages, charge storage is dominated by electric double-layer formation, while at higher biases, intercalation and surface redox reactions contribute additional pseudocapacitance. Studies have demonstrated specific capacitances ranging from 100 to 400 F g⁻¹ depending on the voltage window and electrolyte composition. The use of ionic liquid electrolytes can further extend the operational voltage window, enhancing energy density.

Despite these advantages, ambient instability remains a critical limitation. BP degrades rapidly in the presence of oxygen and moisture, forming phosphorus oxides that degrade electrochemical performance. Covalent functionalization strategies, as mentioned earlier, provide partial mitigation. Another approach involves hybridizing BP with MXenes, which combine high conductivity with excellent environmental stability. For example, BP-Ti₃C₂Tₓ MXene hybrids exhibit synergistic effects, where MXene sheets prevent BP oxidation while BP enhances the composite's pseudocapacitive contribution. Electrochemical measurements of such hybrids have shown capacitance retention exceeding 90% after 10,000 cycles, a significant improvement over pristine BP electrodes.

Hybrid designs incorporating conductive polymers further enhance performance by combining BP's high capacitance with the mechanical flexibility and conductivity of polymers. Polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) have been successfully integrated with BP through in-situ polymerization or layer-by-layer assembly. These composites exhibit improved rate capability and cycling stability due to the conductive polymer's role in facilitating charge transfer and buffering volume changes during cycling. For instance, BP-PEDOT:PSS electrodes have demonstrated areal capacitances exceeding 200 mF cm⁻² at high current densities.

Scalability and cost remain practical challenges for BP-based supercapacitors. While liquid-phase exfoliation is suitable for lab-scale production, large-area uniformity and yield must be improved for commercialization. Recent advances in electrochemical exfoliation and shear-assisted methods show promise for scalable production. Additionally, the development of low-cost passivation techniques will be crucial for real-world applications.

In summary, few-layer BP offers a compelling combination of high capacitance, anisotropic transport, and tunable electrochemistry for supercapacitor applications. Liquid-phase exfoliation and stabilization techniques have advanced significantly, enabling the production of air-stable BP dispersions. Hybrid designs with MXenes or conductive polymers address stability and performance limitations, pushing the boundaries of energy storage technology. Future research should focus on scalable synthesis, long-term stability under operational conditions, and integration into flexible and wearable devices. With continued progress, BP-based electrodes could play a transformative role in next-generation energy storage systems.