Bioinspired Three-Dimensional Nanoporous Membranes for Salinity-Gradient Energy Harvesting

ConspectusSalinity-gradient energy represents a widespread, clean, environmentally friendly, and sustainable source of renewable energy, which has attracted great attention in the past years. To harness this energy, interdisciplinary efforts from chemistry, materials science, environmental science, and nanotechnology have been made to develop efficient and low-cost approaches and materials for energy conversion. Conventional reverse electrodialysis (RED) systems are generally based on ion-exchange membranes, which usually suffer from ineffective mass transport, high membrane resistance, limited pore size, and concentration polarization, resulting in low output power density and poor energy-conversion efficiency. As one promising material, nanofluidic channels with their unique transport properties, which can be attributed to nanoconfinement effect, enable high-performance reverse electrodialysis to efficiently harvest salinity-gradient energy. Due to the unique porous architectures, three-dimensional (3D) nanoporous membranes demonstrate great potential for harvesting salinity-gradient power. It is generally known that the porous membranes can be prepared by many methods; however, there are some shortcomings such as high costs, poor ion conductance, and fragility limiting the practical application. Several simple and versatile approaches to low-cost fabrication of 3D nanoporous membranes have been developed in recent years. For example, self-assembly provides an effective route of constructing functional materials and organizing them into 3D architectures. In this Account, we mainly review our recent progress in the design and fabrication of bioinspired 3D nanoporous membranes for salinity-gradient energy harvesting. First, we give a brief introduction to bioinspired nanochannel membranes (BNMs) with diverse structural dimensions, and nanofluidic channel membranes may lead to technological breakthroughs and thus act as an emerging platform for harvesting salinity-gradient energy. Subsequently, we discuss the typical preparation approaches for bioinspired 3D nanoporous membranes. To tackle the bottlenecks of the conventional membrane-based power generator and extrapolate single-channel devices to macroscopic materials, our group have developed a series of 3D nanoporous membranes for power generation via various simple and versatile methods. We highlight the design and fabrication of several types of 3D nanoporous membranes, i.e., heterogeneous and homogeneous membranes, with tunable surface charge and porosity. The proof-of-concept demonstration of bioinspired 3D porous membranes shows that these nanofluidic platforms have the potential to overcome the selectivity-permeability trade-off and have impressive osmotic-energy-harvesting performance. Specifically, the scale-up Janus 3D porous membranes maintained high selectivity and rectified current in a hypersaline environment, which benefitted effective energy conversion and high output power density when seawater and river water were mixed. Finally, we give an outlook for future challenges and perspectives on the development of 3D nanofluidics for salinity-gradient energy conversion. We expect that this Account will spark further efforts on the development of bioinspired 3D nanoporous membranes for large-scale (typical side length of more than 10 cm) energy conversion and new opportunities for the applications in water desalination, dialysis, and ionic circuitries.