With rapid fossil fuel consumption and ecological concerns, alternative options of green energy development and its efficient storage technology is an emergent area of research. Nanocellulose is observed to be a very-promising sustainable and environmentally friendly nanomaterial for green and renewable electronics for advanced electrochemical energy conversion/conservation devices. This review begins with a basic introduction on the sources and properties of nanocellulose. It provides an overview of the recent advancements made by researchers in integrating nanocellulose with active materials to form a flexible film/aerogel/3D structures as a substrate for powering portable electronics, electric vehicles, etc. The review highlights the use of nanocellulose-based composites in energy conversion devices such as solar cells, piezoelectric materials, and lithium ion batteries. Recent research shows that the power conversion efficiency of solar cells and the piezoelectric performance of piezoelectric materials can be increased when the matrix is reinforced with nanocellulose. The review also focuses on the updates of nanocellulose-based composites in separators, binders, and electrodes of energy conservation devices such as supercapacitors, and energy capture devices such as CO2 separators. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48959. © 2020 Wiley Periodicals, Inc.