A review on direct osmotic power generation: Mechanism and membranes

The emerging high-efficiency membrane-based direct osmotic power generation (MDOPG) technology can promisingly alleviate energy shortage and environmental pollution due to its eco-friendly energy conversion mechanisms. We conduct a state-of-the-art review on three different research branches of MDOPG, including reverse electrodialysis (RED), nanofluidic reverse electrodialysis (NRED), and diffusio-osmosis (DO), and clarify their interrelations and consistency for the first time by analyzing their universal electrokinetic mechanism. Based on the universal cognition of MDOPG, a systematic and comprehensive classification scheme of all kinds of membranes investigated in MDOPG is proposed to facilitate critical insights into key aspects of membranes, including electrochemical and structural properties, strengths and weaknesses, materials, fabrication technologies, economic evaluations, etc. Moreover, we give an outlook on the challenges and the corresponding potential solutions for future MDOPG research, including enhancing the ion transportation efficiency of membranes, scaling-up membranes, and promising applications. Generally, this in-depth review is intended to provide a full overview of MDOPG from different viewpoints of chemical science, material science, electrokinetics, morphology, mechanics, and economics, which will significantly promote the future development and breakthrough of MDOPG from various perspectives.