Inorganic two-dimensional (2D) materials offer a new approach to control mass diffusion at the nanoscale. Controlling ion transport in nanofluidics is key to energy conversion, energy storage, water purification, and numerous other applications wherein persistent challenges for efficient separation must be addressed. We herein discuss the recent development of 2D membranes in the emerging field of energy harvesting, water desalination, and proton/Li-ion production in the context of green energy and environmental technology. We highlight the fundamental mechanisms, 2D membrane fabrication, and challenges toward practical applications. Finally, we outline the fundamental issues of thermodynamics and kinetics along with potential membrane designs that must be resolved to bridge the gap between lab-scale experiments and production levels. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.