Ion Transport in Nanofluidic Devices for Energy Harvesting

Access to sustainable clean energy is one of the key challenges faced by our modern society. Although several sources of clean energy including solar, wind, and water power have been identified and developed, to date, none of these power sources can replace fossil fuels, mainly because of the limited efficiency and high cost of generating and storing electrical power. Nature can perhaps provide a unique perspective for clean energy generation because energy conversion and storage systems in biology work via ion transport and energy storage molecules in an integrative and effective way. The recent development of ion-transport-based energy conversion systems has attracted more and more attention. The ion passive transport for salinity gradient energy generation has realized power density of approximately 5 W m−2, which has been flagged as the target for making salinity gradient power economically viable. Meanwhile, ion active transport has enough “power” to pump ions against steep concentration gradients up to 5,000-fold and can be used for photoelectric energy conversion. Taking the long view, these ion-transport-based energy-harvesting systems should be considered as a primary method, or at least an efficient supplementary way for clean energy harvesting. In this review, we mainly focus on ion-transport-based energy conversion. Aiming to get a deeper understanding of ion-transport-based energy conversion systems, the operating mechanisms, including ion selectivity and ion rectification, are discussed first. For the ion passive transport for harvesting salinity gradient energy, the specific features and power density of 1D/2D/3D nanofluidics are summarized. For the ion active transport for solar energy generation, three preliminary approaches and their derived concepts, including pseudo-ion pump/physical ion pump/chemical ion pump, are proposed. Finally, future ion transport energy-harvesting devices, opportunities, and challenges are speculated upon. Our technological systems are mainly based on semiconductor photovoltaics, electronic circuits, and (electro)chemical storage reactions. However, in the energy field, “ionics” has the potential to complement “electronics.” The control of ion transport is a necessary condition for the existence of life, e.g., both the energy conversion into ATP and the energy consumption to regulate biological functions occurs via directed ion or proton transport. These processes can be mimicked in synthetic devices and (nano)machines and then used for energy harvesting. This review will discuss and summarize the state of the art in the field of ion-transport-based energy conversion systems including ion passive transport for salinity gradient energy conversion and ion active transport for solar energy harvesting and then venture to propose several potential strategies to construct ion transport (passive or active) systems for energy conversion and storage devices, which are useful to drive local chemical reactions or electric current generation. Our technological systems are mainly based on semiconductor photovoltaics, electronic circuits, and (electro)chemical storage reactions. However, in the energy field, “ionics” has the potential to complement “electronics.” The control of ion transport is a necessary condition for the existence of life, e.g., both the energy conversion into ATP and the energy consumption to regulate biological functions occurs via directed ion or proton transport. These processes can be mimicked in synthetic devices and (nano)machines and then used for energy harvesting. This review will discuss and summarize the state of the art in the field of ion-transport-based energy conversion systems including ion passive transport for salinity gradient energy conversion and ion active transport for solar energy harvesting and then venture to propose several potential strategies to construct ion transport (passive or active) systems for energy conversion and storage devices, which are useful to drive local chemical reactions or electric current generation.