Abstract
Due to the capricious nature of renewable energy resources, such as wind and solar, large-scale energy storage devices are increasingly required to make the best use of the renewable power. The redox flow battery is considered suitable for large-scale applications due to its modular design, good scalability and flexible operation. The biggest challenge of the redox flow battery is the low energy density. The redox active species is the most important component in redox flow batteries, and the redox potential and solubility of redox species dictate the system energy density. This review is focused on the recent development of redox species. Different categories of redox species, including simple inorganic ions, metal complexes, metal-free organic compounds, polysulfide/sulfur and lithium storage active materials, are reviewed. The future development of redox species towards higher energy density is also suggested.
Highlights
To meet the energy demands for the increasing global population, the present energy production of the world will be doubled by 2050 [1]
The recent development of aqueous redox flow battery (RFB) involves a wider range of redox species
The solubility of pristine ferrocene is still far too low to be considered as an energy storage species in a redox flow battery
Summary
To meet the energy demands for the increasing global population, the present energy production of the world will be doubled by 2050 [1]. Due to limited fossil fuel resources and global climate change, the energy increase must be achieved without dramatic CO2 emissions [2] Under this goal, renewable energy technologies have been popular topics in recent decades. The conversion between chemical energy and electrical energy occurs while the electrolytes are flowing through the electrodes [6] Such a configuration brings RFB the most unique and attractive feature that other battery systems do not have: decoupled energy storage and power generation, Molecules 2015, 20, 20499–20517; doi:10.3390/molecules201119711 www.mdpi.com/journal/molecules. Molecules 2015, 20, 20499–20517 Molecules 2015, 20, page–page which enables an independent control of capacity and power to meet different requirements ranging from a few kWh too sseevveerraall MMWWhh. The redox flflow battery has been developed for large-scale application for decades, and some of the RFB systems have been successfully demonstraatteedd att the meggaawwaatttt ssccaallee. With an overview of different categories of redox species, a trend towards future higher energy density can be suggested
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