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1. Principles of operation for redox flow batteries Electrochemical energy storage (EES) devices store energy for later use. To clearly showcase the unique features of redox flow batteries (RFBs), we will contrast them and their principles with three other devices: Electrochemical double layer supercapacitors (EDLSCs), solid-state batteries (SSBs) and fuel cells (FCs). These four archetypes capitalize on different physical and chemical principles, not even all of them employ redox reactions to store energy, and are therefore suitable for diverse EES applications. Examples for these applications are balancing and stabilizing the grid [1], electric vehicles [2,3] and mobile consumer electronics [4,5]. Schematics of these four devices are shown in Fig. 1. EDLSCs (Fig. 1a) work on the principle that a U at an electrode- electrolyte interface, which establishes itself due to the different Fermi energies of (metal) electrode EF,Me and electrolyte EF,EL, is balanced by accumulating charges. Cations or anions in the electrolyte form an electrochemical double layer, conglomerating in order to keep the electrochemical potential across the interface constant [6,7]. Charges in the electrode, counterbalanced by charges in the electrolyte form a capacitor with a differential capacitance Cd equal to: (1) With accumulated charge Q, distance between charges d, dielectric constant , permittivity of free space and electrochemically active surface area A. High specific capacitances of 10 10-6 F cm-2 are reported in the literature for carbon electrodes in aqueous electrolytes [8], because the first layer of charges is very close to the electrode (some Ångströms). Carbon can have a high gravimetric surface area ( 1000 m g-1 ) [9 11]. The energy ESC of an EDLSC, which is stored entirely in the electrochemical double layer, is given by: (2) The maximum voltage is limited by the oxidative and reductive stability of the system comprising electrodes and electrolyte. Organic electrolytes and room temperature ionic liquids can sustain higher voltages than water (approx. 1.23 V) [12,13]. In commercial EDLSCs propylene carbonate or acetonitrile are used as solvent with quaternary ammonium Page 4 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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