Oxidation promoted osmotic energy conversion in black phosphorus membranes

Two-dimensional (2D) nanofluidic ion transporting membranes show great promise in harvesting the "blue" osmotic energy between river water and sea water. Black phosphorus (BP), an emerging layered material, has recently been explored for a wide range of ambient applications. However, little attention has been paid to the extraction of the worldwide osmotic energy, despite its large potential as an energy conversion membrane. Here, we report an experimental investigation of BP membrane in osmotic energy conversion and reveal how the oxidation of BP influences power generation. Through controllable oxidation in water, power output of the BP membrane can be largely enhanced, which can be attributed to the generated charged phosphorus compounds. Depending on the valence of oxidized BP that is associated with oxygen concentration, the power density can be precisely controlled and substantially promoted by ∼220% to 1.6 W/m² (compared with the pristine BP membrane). Moreover, through constructing a heterostructure with graphene oxide, ion selectivity of the BP membrane increases by ∼80%, contributing to enhanced charge separation efficiency and thus improved performance of ∼4.7 W/m² that outperforms most of the state-of-the-art 2D nanofluidic membranes.