Nitrogen fixation is a critical process for producing ammonia, a key component in fertilizers and industrial chemicals. Traditional methods like the Haber-Bosch process require high temperatures and pressures, consuming significant energy and emitting greenhouse gases. As an alternative, catalytic nitrogen reduction reaction (NRR) under ambient conditions offers a sustainable pathway. Among emerging materials, phosphorene, a two-dimensional form of black phosphorus, has shown promise due to its unique structural and electronic properties that facilitate nitrogen activation and conversion.

Phosphorene possesses a puckered honeycomb lattice, distinct from flat graphene, which creates anisotropic electronic and mechanical behavior. The inherent structural asymmetry leads to high carrier mobility and tunable bandgap, ranging from 0.3 eV in bulk to around 2.0 eV in monolayer form. This direct bandgap enables efficient light absorption and charge transfer, beneficial for photocatalytic and electrocatalytic NRR. The lone-pair electrons on phosphorus atoms further enhance surface reactivity, providing active sites for nitrogen adsorption and activation. Unlike graphene, phosphorene’s non-planar geometry induces localized charge density at its edges, which are highly reactive toward N2 molecules. Studies indicate that zigzag edges exhibit stronger binding to nitrogen than armchair edges, promoting dissociation of the N≡N triple bond.

However, phosphorene faces challenges in NRR due to the competing hydrogen evolution reaction (HER), which reduces Faradaic efficiency. Strategies to suppress HER include modulating the electronic structure through doping and defect engineering. Transition metal dopants like Fe, Mo, and Ru have been explored to enhance nitrogen adsorption while weakening hydrogen binding. For instance, Fe-doped phosphorene demonstrates a lower overpotential for NRR compared to pristine phosphorene, as the d-orbitals of Fe hybridize with nitrogen’s antibonding orbitals. Vacancy engineering, such as introducing phosphorus vacancies, creates unsaturated sites that preferentially adsorb N2 over protons. Computational studies suggest that single vacancies can lower the energy barrier for N2 dissociation, improving catalytic activity.

Heterojunction designs further optimize phosphorene’s performance by leveraging interfacial charge transfer. Coupling phosphorene with metal oxides (e.g., TiO2) or sulfides (e.g., MoS2) forms built-in electric fields that direct electrons toward nitrogen reduction. In such systems, phosphorene acts as an electron reservoir, while the co-catalyst stabilizes reaction intermediates. For example, a phosphorene-MoS2 heterostructure exhibits enhanced NRR activity due to synergistic effects, where MoS2 provides sulfur-edge sites for protonation while phosphorene facilitates electron donation. Similarly, combining phosphorene with graphene derivatives improves conductivity and prevents aggregation, maintaining accessible active sites.

In-situ characterization techniques are essential to identify active sites and monitor reaction mechanisms. Operando X-ray absorption spectroscopy (XAS) reveals changes in the oxidation state of dopants during catalysis, while in-situ Raman spectroscopy tracks structural evolution under reaction conditions. Electrochemical impedance spectroscopy (EIS) helps elucidate charge transfer kinetics, correlating interfacial properties with catalytic performance. These methods confirm that edge sites and defects dominate NRR activity, guiding material optimization.

Despite its potential, phosphorene suffers from ambient instability due to oxidation, which degrades its catalytic properties. Encapsulation with inert layers like hexagonal boron nitride (hBN) or polymer coatings mitigates degradation while preserving reactivity. Scalability remains another challenge, as large-area synthesis of defect-free phosphorene is difficult. Solution-phase exfoliation and chemical vapor deposition (CVD) are being refined to produce high-quality films, but reproducibility and yield need improvement.

Future efforts should focus on stabilizing phosphorene without compromising its catalytic activity. Advanced doping strategies, such as dual-atom catalysts, could further tailor electronic properties for selective NRR. Integrating phosphorene into hybrid systems with molecular catalysts may also enhance efficiency. Addressing these challenges will advance phosphorene-based NRR toward practical applications, contributing to sustainable ammonia production.

In summary, phosphorene’s anisotropic structure, tunable bandgap, and edge reactivity make it a compelling candidate for ambient NRR. By suppressing HER through doping, vacancy engineering, and heterojunction design, its catalytic performance can be optimized. In-situ characterization provides critical insights into active sites and mechanisms, while stability and scalability require ongoing innovation. With continued research, phosphorene could play a pivotal role in green nitrogen fixation technologies.