Hydrovoltaic Electricity Generator with Hygroscopic Materials: A Review and New Perspective.

The global energy crisis caused by the overconsumption of non-renewable fuels has prompted researchers to develop alternative strategies for producing electrical energy. In this review, we introduce a fascinating strategy that simply utilizes water, an abundant natural substance throughout the globe and even in air as moisture, as a power source. The concept of the hydrovoltaic electricity generator (HEG) proposed herein involves generating an electrical potential gradient by exposing the two ends of the HEG device to dissimilar physicochemical environments, which leads to the production of an electrical current through the active material. HEGs, with a large variety of viable active materials, have much potential for expansion toward diverse applications including permanent and/or emergency power sources. In this review, we discuss representative HEGs that generate electricity by the mechanisms of diffusion, streaming, and capacitance as case studies for building a fundamental understanding of the electricity generation process. In particular, by comparing the use and absence of hygroscopic materials, we meticulously elucidate HEG mechanism studies to establish active material design principles. We conclude the review with future perspectives on electrode design using conducting nanomaterials, considerations for high performance device construction, and potential impacts of the HEG technology in improving our livelihoods. This article is protected by copyright. All rights reserved.