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Abstract
Based on inexpensive, safe, and environmentally friendly active redox species, neutral polysulfide-ferrocyanide redox flow batteries (PFRFBs) have attracted much attention for large-scale energy storage. However, the development of PFRFBs is undermined by the expensive commercial membrane materials as well as the sluggish polysulfide redox reactions. This work attempts to solve these critical problems by combining the economical membrane with the highly catalytic electrode. In specific, K + -exchanged sulfonated polyether ether ketone (SPEEK-K) membranes have been investigated in PFRFBs to replace the costly Nafion membrane. SPEEK-K with optimized degree of sulfonation enables the PFRFB high average coulombic efficiency of 99.80% and superior energy efficiency of 90.42% at a current density of 20 mA cm -2 . Meanwhile, to overcome the kinetic limitations of polysulfide redox reactions, a CuS-modified carbon felt electrode is demonstrated with excellent catalytic performance, enabling the PFRFB higher and more stable energy efficiency over cycling. The combination of the cost-effective membrane with the catalytic electrode in one cell leads to a capacity retention of 99.54% after 1180 cycles and an outstanding power density (up to 223 mW cm -2 ). The significant enhancements of electrochemical performance at reduced capital cost will make the PFRFB more promising for large-scale energy storage systems.
Highlights
The increasing severity of climate change is pushing a global energy transition from fossil fuels to renewable energy sources such as solar and wind
To overcome the kinetic limitations of polysulfide redox reactions, a CuS-modified carbon felt electrode is demonstrated with excellent catalytic performance, enabling the polysulfide-ferrocyanide redox flow batteries (PFRFBs) higher and more stable energy efficiency over cycling
The homogeneous distributions of K+ ions into the matrixes are verified by the energy-dispersive spectrometer (EDS) mapping results (Figures 2(b)–2(d))
Summary
The increasing severity of climate change is pushing a global energy transition from fossil fuels to renewable energy sources such as solar and wind. To facilitate the energy transition, the energy storage technologies are required to be efficient, safe, and affordable [1,2,3]. The design of redox-active organic materials with desirable electrochemical performances remains challenging [12]. To this end, many efforts have been directed to the investigations of new RFBs to achieve low cost [13,14,15], high energy density [16,17,18,19], low capacitance decay rate [20,21,22], and long lifetime [23,24,25,26,27,28,29]
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